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lvm2/man/lvmraid.7_main
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.TH "LVMRAID" "7" "LVM TOOLS #VERSION#" "Red Hat, Inc" "\""
.SH NAME
lvmraid \(em LVM RAID
.SH DESCRIPTION
\fBlvm\fP(8) RAID is a way to create a Logical Volume (LV) that uses
multiple physical devices to improve performance or tolerate device
failures. In LVM, the physical devices are Physical Volumes (PVs) in a
single Volume Group (VG).
How LV data blocks are placed onto PVs is determined by the RAID level.
RAID levels are commonly referred to as 'raid' followed by a number, e.g.
raid1, raid5 or raid6. Selecting a RAID level involves making tradeoffs
among: physical device requirements, fault tolerance, and performance. A
description of the RAID levels can be found at
.br
www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf
LVM RAID uses both Device Mapper (DM) and Multiple Device (MD) drivers
from the Linux kernel. DM is used to create and manage visible LVM
devices, and MD is used to place data on physical devices.
LVM creates hidden LVs (dm devices) layered between the visible LV and
physical devices. LVs in the middle layers are called sub LVs.
For LVM raid, a sub LV pair to store data and metadata (raid superblock
and write intent bitmap) is created per raid image/leg (see lvs command examples below).
.SH Create a RAID LV
To create a RAID LV, use lvcreate and specify an LV type.
The LV type corresponds to a RAID level.
The basic RAID levels that can be used are:
.B raid0, raid1, raid4, raid5, raid6, raid10.
.B lvcreate --type
.I RaidLevel
[\fIOPTIONS\fP]
.B --name
.I Name
.B --size
.I Size
.I VG
[\fIPVs\fP]
To display the LV type of an existing LV, run:
.B lvs -o name,segtype
\fILV\fP
(The LV type is also referred to as "segment type" or "segtype".)
LVs can be created with the following types:
.SS raid0
\&
Also called striping, raid0 spreads LV data across multiple devices in
units of stripe size. This is used to increase performance. LV data will
be lost if any of the devices fail.
.B lvcreate --type raid0
[\fB--stripes\fP \fINumber\fP \fB--stripesize\fP \fISize\fP]
\fIVG\fP
[\fIPVs\fP]
.HP
.B --stripes
specifies the number of devices to spread the LV across.
.HP
.B --stripesize
specifies the size of each stripe in kilobytes. This is the amount of
data that is written to one device before moving to the next.
.P
\fIPVs\fP specifies the devices to use. If not specified, lvm will choose
\fINumber\fP devices, one for each stripe based on the number of PVs
available or supplied.
.SS raid1
\&
Also called mirroring, raid1 uses multiple devices to duplicate LV data.
The LV data remains available if all but one of the devices fail.
The minimum number of devices (i.e. sub LV pairs) required is 2.
.B lvcreate --type raid1
[\fB--mirrors\fP \fINumber\fP]
\fIVG\fP
[\fIPVs\fP]
.HP
.B --mirrors
specifies the number of mirror images in addition to the original LV
image, e.g. --mirrors 1 means there are two images of the data, the
original and one mirror image.
.P
\fIPVs\fP specifies the devices to use. If not specified, lvm will choose
\fINumber\fP devices, one for each image.
.SS raid4
\&
raid4 is a form of striping that uses an extra, first device dedicated to
storing parity blocks. The LV data remains available if one device fails. The
parity is used to recalculate data that is lost from a single device. The
minimum number of devices required is 3.
.B lvcreate --type raid4
[\fB--stripes\fP \fINumber\fP \fB--stripesize\fP \fISize\fP]
\fIVG\fP
[\fIPVs\fP]
.HP
.B --stripes
specifies the number of devices to use for LV data. This does not include
the extra device lvm adds for storing parity blocks. A raid4 LV with
\fINumber\fP stripes requires \fINumber\fP+1 devices. \fINumber\fP must
be 2 or more.
.HP
.B --stripesize
specifies the size of each stripe in kilobytes. This is the amount of
data that is written to one device before moving to the next.
.P
\fIPVs\fP specifies the devices to use. If not specified, lvm will choose
\fINumber\fP+1 separate devices.
raid4 is called non-rotating parity because the parity blocks are always
stored on the same device.
.SS raid5
\&
raid5 is a form of striping that uses an extra device for storing parity
blocks. LV data and parity blocks are stored on each device, typically in
a rotating pattern for performance reasons. The LV data remains available
if one device fails. The parity is used to recalculate data that is lost
from a single device. The minimum number of devices required is 3 (unless
converting from 2 legged raid1 to reshape to more stripes; see reshaping).
.B lvcreate --type raid5
[\fB--stripes\fP \fINumber\fP \fB--stripesize\fP \fISize\fP]
\fIVG\fP
[\fIPVs\fP]
.HP
.B --stripes
specifies the number of devices to use for LV data. This does not include
the extra device lvm adds for storing parity blocks. A raid5 LV with
\fINumber\fP stripes requires \fINumber\fP+1 devices. \fINumber\fP must
be 2 or more.
.HP
.B --stripesize
specifies the size of each stripe in kilobytes. This is the amount of
data that is written to one device before moving to the next.
.P
\fIPVs\fP specifies the devices to use. If not specified, lvm will choose
\fINumber\fP+1 separate devices.
raid5 is called rotating parity because the parity blocks are placed on
different devices in a round-robin sequence. There are variations of
raid5 with different algorithms for placing the parity blocks. The
default variant is raid5_ls (raid5 left symmetric, which is a rotating
parity 0 with data restart.) See \fBRAID5 variants\fP below.
.SS raid6
\&
raid6 is a form of striping like raid5, but uses two extra devices for
parity blocks. LV data and parity blocks are stored on each device, typically
in a rotating pattern for perfomramce reasons. The
LV data remains available if up to two devices fail. The parity is used
to recalculate data that is lost from one or two devices. The minimum
number of devices required is 5.
.B lvcreate --type raid6
[\fB--stripes\fP \fINumber\fP \fB--stripesize\fP \fISize\fP]
\fIVG\fP
[\fIPVs\fP]
.HP
.B --stripes
specifies the number of devices to use for LV data. This does not include
the extra two devices lvm adds for storing parity blocks. A raid6 LV with
\fINumber\fP stripes requires \fINumber\fP+2 devices. \fINumber\fP must be
3 or more.
.HP
.B --stripesize
specifies the size of each stripe in kilobytes. This is the amount of
data that is written to one device before moving to the next.
.P
\fIPVs\fP specifies the devices to use. If not specified, lvm will choose
\fINumber\fP+2 separate devices.
Like raid5, there are variations of raid6 with different algorithms for
placing the parity blocks. The default variant is raid6_zr (raid6 zero
restart, aka left symmetric, which is a rotating parity 0 with data
restart.) See \fBRAID6 variants\fP below.
.SS raid10
\&
raid10 is a combination of raid1 and raid0, striping data across mirrored
devices. LV data remains available if one or more devices remains in each
mirror set. The minimum number of devices required is 4.
.B lvcreate --type raid10
.RS
[\fB--mirrors\fP \fINumberMirrors\fP]
.br
[\fB--stripes\fP \fINumberStripes\fP \fB--stripesize\fP \fISize\fP]
.br
\fIVG\fP
[\fIPVs\fP]
.RE
.HP
.B --mirrors
specifies the number of mirror images within each stripe. e.g.
--mirrors 1 means there are two images of the data, the original and one
mirror image.
.HP
.B --stripes
specifies the total number of devices to use in all raid1 images (not the
number of raid1 devices to spread the LV across, even though that is the
effective result). The number of devices in each raid1 mirror will be
NumberStripes/(NumberMirrors+1), e.g. mirrors 1 and stripes 4 will stripe
data across two raid1 mirrors, where each mirror is devices.
.HP
.B --stripesize
specifies the size of each stripe in kilobytes. This is the amount of
data that is written to one device before moving to the next.
.P
\fIPVs\fP specifies the devices to use. If not specified, lvm will choose
the necessary devices. Devices are used to create mirrors in the
order listed, e.g. for mirrors 1, stripes 2, listing PV1 PV2 PV3 PV4
results in mirrors PV1/PV2 and PV3/PV4.
RAID10 is not mirroring on top of stripes, which would be RAID01, which is
less tolerant of device failures.
.SH Synchronization
Synchronization is the process that makes all the devices in a RAID LV
consistent with each other.
In a RAID1 LV, all mirror images should have the same data. When a new
mirror image is added, or a mirror image is missing data, then images need
to be synchronized. Data blocks are copied from an existing image to a
new or outdated image to make them match.
In a RAID 4/5/6 LV, parity blocks and data blocks should match based on
the parity calculation. When the devices in a RAID LV change, the data
and parity blocks can become inconsistent and need to be synchronized.
Correct blocks are read, parity is calculated, and recalculated blocks are
written.
The RAID implementation keeps track of which parts of a RAID LV are
synchronized. When a RAID LV is first created and activated the first
synchronization is called initialization. A pointer stored in the raid
metadata keeps track of the initialization process thus allowing it to be
restarted after a deactivation of the RaidLV or a crash. Any writes to
the RaidLV dirties the respective region of the write intent bitmap which
allow for fast recovery of the regions after a crash. Without this, the
entire LV would need to be synchronized every time it was activated.
Automatic synchronization happens when a RAID LV is activated, but it is
usually partial because the bitmaps reduce the areas that are checked.
A full sync becomes necessary when devices in the RAID LV are replaced.
The synchronization status of a RAID LV is reported by the
following command, where "Cpy%Sync" = "100%" means sync is complete:
.B lvs -a -o name,sync_percent
.SS Scrubbing
Scrubbing is a full scan of the RAID LV requested by a user.
Scrubbing can find problems that are missed by partial synchronization.
Scrubbing assumes that RAID metadata and bitmaps may be inaccurate, so it
verifies all RAID metadata, LV data, and parity blocks. Scrubbing can
find inconsistencies caused by hardware errors or degradation. These
kinds of problems may be undetected by automatic synchronization which
excludes areas outside of the RAID write-intent bitmap.
The command to scrub a RAID LV can operate in two different modes:
.B lvchange --syncaction
.BR check | repair
.I LV
.HP
.B check
Check mode is read-only and only detects inconsistent areas in the RAID
LV, it does not correct them.
.HP
.B repair
Repair mode checks and writes corrected blocks to synchronize any
inconsistent areas.
.P
Scrubbing can consume a lot of bandwidth and slow down application I/O on
the RAID LV. To control the I/O rate used for scrubbing, use:
.HP
.B --maxrecoveryrate
\fISize\fP[k|UNIT]
.br
Sets the maximum recovery rate for a RAID LV. \fISize\fP is specified as
an amount per second for each device in the array. If no suffix is given,
then KiB/sec/device is used. Setting the recovery rate to \fB0\fP
means it will be unbounded.
.HP
.BR --minrecoveryrate
\fISize\fP[k|UNIT]
.br
Sets the minimum recovery rate for a RAID LV. \fISize\fP is specified as
an amount per second for each device in the array. If no suffix is given,
then KiB/sec/device is used. Setting the recovery rate to \fB0\fP
means it will be unbounded.
.P
To display the current scrubbing in progress on an LV, including
the syncaction mode and percent complete, run:
.B lvs -a -o name,raid_sync_action,sync_percent
After scrubbing is complete, to display the number of inconsistent blocks
found, run:
.B lvs -o name,raid_mismatch_count
Also, if mismatches were found, the lvs attr field will display the letter
"m" (mismatch) in the 9th position, e.g.
.nf
# lvs -o name,vgname,segtype,attr vg/lv
LV VG Type Attr
lv vg raid1 Rwi-a-r-m-
.fi
.SS Scrubbing Limitations
The \fBcheck\fP mode can only report the number of inconsistent blocks, it
cannot report which blocks are inconsistent. This makes it impossible to
know which device has errors, or if the errors affect file system data,
metadata or nothing at all.
The \fBrepair\fP mode can make the RAID LV data consistent, but it does
not know which data is correct. The result may be consistent but
incorrect data. When two different blocks of data must be made
consistent, it chooses the block from the device that would be used during
RAID intialization. However, if the PV holding corrupt data is known,
lvchange --rebuild can be used in place of scrubbing to reconstruct the
data on the bad device.
Future developments might include:
Allowing a user to choose the correct version of data during repair.
Using a majority of devices to determine the correct version of data to
use in a 3-way RAID1 or RAID6 LV.
Using a checksumming device to pin-point when and where an error occurs,
allowing it to be rewritten.
.SH SubLVs
An LV is often a combination of other hidden LVs called SubLVs. The
SubLVs either use physical devices, or are built from other SubLVs
themselves. SubLVs hold LV data blocks, RAID parity blocks, and RAID
metadata. SubLVs are generally hidden, so the lvs -a option is required
to display them:
.B lvs -a -o name,segtype,devices
SubLV names begin with the visible LV name, and have an automatic suffix
indicating its role:
.IP \(bu 3
SubLVs holding LV data or parity blocks have the suffix _rimage_#.
These SubLVs are sometimes referred to as DataLVs.
.IP \(bu 3
SubLVs holding RAID metadata have the suffix _rmeta_#. RAID metadata
includes superblock information, RAID type, bitmap, and device health
information. These SubLVs are sometimes referred to as MetaLVs.
.P
SubLVs are an internal implementation detail of LVM. The way they are
used, constructed and named may change.
The following examples show the SubLV arrangement for each of the basic
RAID LV types, using the fewest number of devices allowed for each.
.SS Examples
.B raid0
.br
Each rimage SubLV holds a portion of LV data. No parity is used.
No RAID metadata is used.
.nf
# lvcreate --type raid0 --stripes 2 --name lvr0 ...
# lvs -a -o name,segtype,devices
lvr0 raid0 lvr0_rimage_0(0),lvr0_rimage_1(0)
[lvr0_rimage_0] linear /dev/sda(...)
[lvr0_rimage_1] linear /dev/sdb(...)
.fi
.B raid1
.br
Each rimage SubLV holds a complete copy of LV data. No parity is used.
Each rmeta SubLV holds RAID metadata.
.nf
# lvcreate --type raid1 --mirrors 1 --name lvr1 ...
# lvs -a -o name,segtype,devices
lvr1 raid1 lvr1_rimage_0(0),lvr1_rimage_1(0)
[lvr1_rimage_0] linear /dev/sda(...)
[lvr1_rimage_1] linear /dev/sdb(...)
[lvr1_rmeta_0] linear /dev/sda(...)
[lvr1_rmeta_1] linear /dev/sdb(...)
.fi
.B raid4
.br
At least three rimage SubLVs each hold a portion of LV data and one rimage SubLV
holds parity. Each rmeta SubLV holds RAID metadata.
.nf
# lvcreate --type raid4 --stripes 2 --name lvr4 ...
# lvs -a -o name,segtype,devices
lvr4 raid4 lvr4_rimage_0(0),\\
lvr4_rimage_1(0),\\
lvr4_rimage_2(0)
[lvr4_rimage_0] linear /dev/sda(...)
[lvr4_rimage_1] linear /dev/sdb(...)
[lvr4_rimage_2] linear /dev/sdc(...)
[lvr4_rmeta_0] linear /dev/sda(...)
[lvr4_rmeta_1] linear /dev/sdb(...)
[lvr4_rmeta_2] linear /dev/sdc(...)
.fi
.B raid5
.br
At least three rimage SubLVs each typcially hold a portion of LV data and parity
(see section on raid5)
Each rmeta SubLV holds RAID metadata.
.nf
# lvcreate --type raid5 --stripes 2 --name lvr5 ...
# lvs -a -o name,segtype,devices
lvr5 raid5 lvr5_rimage_0(0),\\
lvr5_rimage_1(0),\\
lvr5_rimage_2(0)
[lvr5_rimage_0] linear /dev/sda(...)
[lvr5_rimage_1] linear /dev/sdb(...)
[lvr5_rimage_2] linear /dev/sdc(...)
[lvr5_rmeta_0] linear /dev/sda(...)
[lvr5_rmeta_1] linear /dev/sdb(...)
[lvr5_rmeta_2] linear /dev/sdc(...)
.fi
.B raid6
.br
At least five rimage SubLVs each typically hold a portion of LV data and parity.
(see section on raid6)
Each rmeta SubLV holds RAID metadata.
.nf
# lvcreate --type raid6 --stripes 3 --name lvr6
# lvs -a -o name,segtype,devices
lvr6 raid6 lvr6_rimage_0(0),\\
lvr6_rimage_1(0),\\
lvr6_rimage_2(0),\\
lvr6_rimage_3(0),\\
lvr6_rimage_4(0),\\
lvr6_rimage_5(0)
[lvr6_rimage_0] linear /dev/sda(...)
[lvr6_rimage_1] linear /dev/sdb(...)
[lvr6_rimage_2] linear /dev/sdc(...)
[lvr6_rimage_3] linear /dev/sdd(...)
[lvr6_rimage_4] linear /dev/sde(...)
[lvr6_rimage_5] linear /dev/sdf(...)
[lvr6_rmeta_0] linear /dev/sda(...)
[lvr6_rmeta_1] linear /dev/sdb(...)
[lvr6_rmeta_2] linear /dev/sdc(...)
[lvr6_rmeta_3] linear /dev/sdd(...)
[lvr6_rmeta_4] linear /dev/sde(...)
[lvr6_rmeta_5] linear /dev/sdf(...)
.fi
.B raid10
.br
At least four rimage SubLVs each hold a portion of LV data. No parity is used.
Each rmeta SubLV holds RAID metadata.
.nf
# lvcreate --type raid10 --stripes 2 --mirrors 1 --name lvr10
# lvs -a -o name,segtype,devices
lvr10 raid10 lvr10_rimage_0(0),\\
lvr10_rimage_1(0),\\
lvr10_rimage_2(0),\\
lvr10_rimage_3(0)
[lvr10_rimage_0] linear /dev/sda(...)
[lvr10_rimage_1] linear /dev/sdb(...)
[lvr10_rimage_2] linear /dev/sdc(...)
[lvr10_rimage_3] linear /dev/sdd(...)
[lvr10_rmeta_0] linear /dev/sda(...)
[lvr10_rmeta_1] linear /dev/sdb(...)
[lvr10_rmeta_2] linear /dev/sdc(...)
[lvr10_rmeta_3] linear /dev/sdd(...)
.fi
.SH Device Failure
Physical devices in a RAID LV can fail or be lost for multiple reasons.
A device could be disconnected, permanently failed, or temporarily
disconnected. The purpose of RAID LVs (levels 1 and higher) is to
continue operating in a degraded mode, without losing LV data, even after
a device fails. The number of devices that can fail without the loss of
LV data depends on the RAID level:
.IP \[bu] 3
RAID0 (striped) LVs cannot tolerate losing any devices. LV data will be
lost if any devices fail.
.IP \[bu] 3
RAID1 LVs can tolerate losing all but one device without LV data loss.
.IP \[bu] 3
RAID4 and RAID5 LVs can tolerate losing one device without LV data loss.
.IP \[bu] 3
RAID6 LVs can tolerate losing two devices without LV data loss.
.IP \[bu] 3
RAID10 is variable, and depends on which devices are lost. It stripes
across multiple mirror groups with raid1 layout thus it can tolerate
losing all but one device in each of these groups without LV data loss.
.P
If a RAID LV is missing devices, or has other device-related problems, lvs
reports this in the health_status (and attr) fields:
.B lvs -o name,lv_health_status
.B partial
.br
Devices are missing from the LV. This is also indicated by the letter "p"
(partial) in the 9th position of the lvs attr field.
.B refresh needed
.br
A device was temporarily missing but has returned. The LV needs to be
refreshed to use the device again (which will usually require
partial synchronization). This is also indicated by the letter "r" (refresh
needed) in the 9th position of the lvs attr field. See
\fBRefreshing an LV\fP. This could also indicate a problem with the
device, in which case it should be be replaced, see
\fBReplacing Devices\fP.
.B mismatches exist
.br
See
.BR Scrubbing .
Most commands will also print a warning if a device is missing, e.g.
.br
.nf
WARNING: Device for PV uItL3Z-wBME-DQy0-... not found or rejected ...
.fi
This warning will go away if the device returns or is removed from the
VG (see \fBvgreduce --removemissing\fP).
.SS Activating an LV with missing devices
A RAID LV that is missing devices may be activated or not, depending on
the "activation mode" used in lvchange:
.B lvchange -ay --activationmode
.BR complete | degraded | partial
.I LV
.B complete
.br
The LV is only activated if all devices are present.
.B degraded
.br
The LV is activated with missing devices if the RAID level can
tolerate the number of missing devices without LV data loss.
.B partial
.br
The LV is always activated, even if portions of the LV data are missing
because of the missing device(s). This should only be used to perform
extreme recovery or repair operations.
.BR lvm.conf (5)
.B activation/activation_mode
.br
controls the activation mode when not specified by the command.
The default value is printed by:
.nf
lvmconfig --type default activation/activation_mode
.fi
.SS Replacing Devices
Devices in a RAID LV can be replaced by other devices in the VG. When
replacing devices that are no longer visible on the system, use lvconvert
--repair. When replacing devices that are still visible, use lvconvert
--replace. The repair command will attempt to restore the same number
of data LVs that were previously in the LV. The replace option can be
repeated to replace multiple PVs. Replacement devices can be optionally
listed with either option.
.B lvconvert --repair
.I LV
[\fINewPVs\fP]
.B lvconvert --replace
\fIOldPV\fP
.I LV
[\fINewPV\fP]
.B lvconvert
.B --replace
\fIOldPV1\fP
.B --replace
\fIOldPV2\fP
...
.I LV
[\fINewPVs\fP]
New devices require synchronization with existing devices, see
.BR Synchronization .
.SS Refreshing an LV
Refreshing a RAID LV clears any transient device failures (device was
temporarily disconnected) and returns the LV to its fully redundant mode.
Restoring a device will usually require at least partial synchronization
(see \fBSynchronization\fP). Failure to clear a transient failure results
in the RAID LV operating in degraded mode until it is reactivated. Use
the lvchange command to refresh an LV:
.B lvchange --refresh
.I LV
.nf
# lvs -o name,vgname,segtype,attr,size vg
LV VG Type Attr LSize
lv vg raid1 Rwi-a-r-r- 100.00g
# lvchange --refresh vg/lv
# lvs -o name,vgname,segtype,attr,size vg
LV VG Type Attr LSize
lv vg raid1 Rwi-a-r--- 100.00g
.fi
.SS Automatic repair
If a device in a RAID LV fails, device-mapper in the kernel notifies the
.BR dmeventd (8)
monitoring process (see \fBMonitoring\fP).
dmeventd can be configured to automatically respond using:
.BR lvm.conf (5)
.B activation/raid_fault_policy
Possible settings are:
.B warn
.br
A warning is added to the system log indicating that a device has
failed in the RAID LV. It is left to the user to repair the LV, e.g.
replace failed devices.
.B allocate
.br
dmeventd automatically attempts to repair the LV using spare devices
in the VG. Note that even a transient failure is treated as a permanent
failure under this setting. A new device is allocated and full
synchronization is started.
The specific command run by dmeventd to warn or repair is:
.br
.B lvconvert --repair --use-policies
.I LV
.SS Corrupted Data
Data on a device can be corrupted due to hardware errors without the
device ever being disconnected or there being any fault in the software.
This should be rare, and can be detected (see \fBScrubbing\fP).
.SS Rebuild specific PVs
If specific PVs in a RAID LV are known to have corrupt data, the data on
those PVs can be reconstructed with:
.B lvchange --rebuild
.I PV
.I LV
The rebuild option can be repeated with different PVs to replace the data
on multiple PVs.
.SH Monitoring
When a RAID LV is activated the \fBdmeventd\fP(8) process is started to
monitor the health of the LV. Various events detected in the kernel can
cause a notification to be sent from device-mapper to the monitoring
process, including device failures and synchronization completion (e.g.
for initialization or scrubbing).
The LVM configuration file contains options that affect how the monitoring
process will respond to failure events (e.g. raid_fault_policy). It is
possible to turn on and off monitoring with lvchange, but it is not
recommended to turn this off unless you have a thorough knowledge of the
consequences.
.SH Configuration Options
There are a number of options in the LVM configuration file that affect
the behavior of RAID LVs. The tunable options are listed
below. A detailed description of each can be found in the LVM
configuration file itself.
.br
mirror_segtype_default
.br
raid10_segtype_default
.br
raid_region_size
.br
raid_fault_policy
.br
activation_mode
.SH RAID1 Tuning
A RAID1 LV can be tuned so that certain devices are avoided for reading
while all devices are still written to.
.B lvchange
.BR -- [ raid ] writemostly
\fIPV\fP[\fB:y\fP|\fBn\fP|\fBt\fP]
.I LV
The specified device will be marked as "write mostly", which means that
reading from this device will be avoided, and other devices will be
preferred for reading (unless no other devices are available.) This
minimizes the I/O to the specified device.
If the PV name has no suffix, the write mostly attribute is set. If the
PV name has the suffix \fB:n\fP, the write mostly attribute is cleared,
and the suffix \fB:t\fP toggles the current setting.
The write mostly option can be repeated on the command line to change
multiple devices at once.
To report the current write mostly setting, the lvs attr field will show
the letter "w" in the 9th position when write mostly is set:
.B lvs -a -o name,attr
When a device is marked write mostly, the maximum number of outstanding
writes to that device can be configured. Once the maximum is reached,
further writes become synchronous. When synchronous, a write to the LV
will not complete until writes to all the mirror images are complete.
.B lvchange
.BR -- [ raid ] writebehind
.I Number
.I LV
To report the current write behind setting, run:
.B lvs -o name,raid_write_behind
When write behind is not configured, or set to 0, all LV writes are
synchronous.
.SH RAID Takeover
RAID takeover is converting a RAID LV from one RAID level to another, e.g.
raid5 to raid6. Changing the RAID level is usually done to increase or
decrease resilience to device failures or to restripe LVs. This is done
using lvconvert and specifying the new RAID level as the LV type:
.B lvconvert --type
.I RaidLevel
.I LV
[\fIPVs\fP]
The most common and recommended RAID takeover conversions are:
.HP
\fBlinear\fP to \fBraid1\fP
.br
Linear is a single image of LV data, and
converting it to raid1 adds a mirror image which is a direct copy of the
original linear image.
.HP
\fBstriped\fP/\fBraid0\fP to \fBraid4/5/6\fP
.br
Adding parity devices to a
striped volume results in raid4/5/6.
.P
Unnatural conversions that are not recommended include converting between
striped and non-striped types. This is because file systems often
optimize I/O patterns based on device striping values. If those values
change, it can decrease performance.
Converting to a higher RAID level requires allocating new SubLVs to hold
RAID metadata, and new SubLVs to hold parity blocks for LV data.
Converting to a lower RAID level removes the SubLVs that are no longer
needed.
Conversion often requires full synchronization of the RAID LV (see
\fBSynchronization\fP). Converting to RAID1 requires copying all LV data
blocks to N new images on new devices. Converting to a parity RAID level
requires reading all LV data blocks, calculating parity, and writing the
new parity blocks. Synchronization can take a long time depending on the
throughpout of the devices used and the size of the RaidLV. It can degrade
performance (rate controls also apply to conversion; see
\fB--minrecoveryrate\fP
and
\fB--maxrecoveryrate\fP.)
Warning: though it is possible to create \fBstriped\fP LVs with up to 128 stripes,
a maximum of 64 stripes can be converted to \fBraid0\fP, 63 to \fBraid4/5\fP and
62 to \fBraid6\fP because of the added parity SubLVs.
A \fBstriped\fP LV with a maximum of 32 stripes can be converted to \fBraid10\fP.
.P
The following takeover conversions are currently possible:
.br
.IP \(bu 3
between striped and raid0.
.IP \(bu 3
between linear and raid1.
.IP \(bu 3
between mirror and raid1.
.IP \(bu 3
between raid1 with two images and raid4/5.
.IP \(bu 3
between striped/raid0 and raid4.
.IP \(bu 3
between striped/raid0 and raid5.
.IP \(bu 3
between striped/raid0 and raid6.
.IP \(bu 3
between raid4 and raid5.
.IP \(bu 3
between raid4/raid5 and raid6.
.IP \(bu 3
between striped/raid0 and raid10.
.IP \(bu 3
between striped and raid4.
.SS Indirect conversions
Converting from one raid level to another may require multiple steps,
converting first to intermediate raid levels.
.B linear to raid6
To convert an LV from linear to raid6:
.br
1. convert to raid1 with two images
.br
2. convert to raid5 (internally raid5_ls) with two images
.br
3. convert to raid5 with three or more stripes (reshape)
.br
4. convert to raid6 (internally raid6_ls_6)
.br
5. convert to raid6 (internally raid6_zr, reshape)
The commands to perform the steps above are:
.br
1. lvconvert --type raid1 --mirrors 1 LV
.br
2. lvconvert --type raid5 LV
.br
3. lvconvert --stripes 3 LV
.br
4. lvconvert --type raid6 LV
.br
5. lvconvert --type raid6 LV
The final conversion from raid6_ls_6 to raid6_zr is done to avoid the
potential write/recovery performance reduction in raid6_ls_6 because of
the dedicated parity device. raid6_zr rotates data and parity blocks to
avoid this.
.B linear to striped
To convert an LV from linear to striped:
.br
1. convert to raid1 with two images
.br
2. convert to raid5_n
.br
3. convert to raid5_n with five 128k stripes (reshape)
.br
4. convert raid5_n to striped
The commands to perform the steps above are:
.br
1. lvconvert --type raid1 --mirrors 1 LV
.br
2. lvconvert --type raid5_n LV
.br
3. lvconvert --stripes 5 --stripesize 128k LV
.br
4. lvconvert --type striped LV
The raid5_n type in step 2 is used because it has dedicated parity SubLVs
at the end, and can be converted to striped directly. The stripe size is
increased in step 3 to add extra space for the conversion process. This
step grows the LV size by a factor of five. After conversion, this extra
space can be reduced (or used to grow the file system using the LV).
Reversing these steps will convert a striped LV to linear.
.B raid6 to striped
To convert an LV from raid6_nr to striped:
.br
1. convert to raid6_n_6
.br
2. convert to striped
The commands to perform the steps above are:
.br
1. lvconvert --type raid6_n_6 LV
.br
2. lvconvert --type striped LV
.SS Examples
Converting an LV from \fBlinear\fP to \fBraid1\fP.
.nf
# lvs -a -o name,segtype,size vg
LV Type LSize
lv linear 300.00g
# lvconvert --type raid1 --mirrors 1 vg/lv
# lvs -a -o name,segtype,size vg
LV Type LSize
lv raid1 300.00g
[lv_rimage_0] linear 300.00g
[lv_rimage_1] linear 300.00g
[lv_rmeta_0] linear 3.00m
[lv_rmeta_1] linear 3.00m
.fi
Converting an LV from \fBmirror\fP to \fBraid1\fP.
.nf
# lvs -a -o name,segtype,size vg
LV Type LSize
lv mirror 100.00g
[lv_mimage_0] linear 100.00g
[lv_mimage_1] linear 100.00g
[lv_mlog] linear 3.00m
# lvconvert --type raid1 vg/lv
# lvs -a -o name,segtype,size vg
LV Type LSize
lv raid1 100.00g
[lv_rimage_0] linear 100.00g
[lv_rimage_1] linear 100.00g
[lv_rmeta_0] linear 3.00m
[lv_rmeta_1] linear 3.00m
.fi
Converting an LV from \fBlinear\fP to \fBraid1\fP (with 3 images).
.nf
# lvconvert --type raid1 --mirrors 2 vg/lv
.fi
Converting an LV from \fBstriped\fP (with 4 stripes) to \fBraid6_n_6\fP.
.nf
# lvcreate --stripes 4 -L64M -n lv vg
# lvconvert --type raid6 vg/lv
# lvs -a -o lv_name,segtype,sync_percent,data_copies
LV Type Cpy%Sync #Cpy
lv raid6_n_6 100.00 3
[lv_rimage_0] linear
[lv_rimage_1] linear
[lv_rimage_2] linear
[lv_rimage_3] linear
[lv_rimage_4] linear
[lv_rimage_5] linear
[lv_rmeta_0] linear
[lv_rmeta_1] linear
[lv_rmeta_2] linear
[lv_rmeta_3] linear
[lv_rmeta_4] linear
[lv_rmeta_5] linear
.fi
This convert begins by allocating MetaLVs (rmeta_#) for each of the
existing stripe devices. It then creates 2 additional MetaLV/DataLV pairs
(rmeta_#/rimage_#) for dedicated raid6 parity.
If rotating data/parity is required, such as with raid6_nr, it must be
done by reshaping (see below).
.SH RAID Reshaping
RAID reshaping is changing attributes of a RAID LV while keeping the same
RAID level. This includes changing RAID layout, stripe size, or number of
stripes.
When changing the RAID layout or stripe size, no new SubLVs (MetaLVs or
DataLVs) need to be allocated, but DataLVs are extended by a small amount
(typically 1 extent). The extra space allows blocks in a stripe to be
updated safely, and not be corrupted in case of a crash. If a crash occurs,
reshaping can just be restarted.
(If blocks in a stripe were updated in place, a crash could leave them
partially updated and corrupted. Instead, an existing stripe is quiesced,
read, changed in layout, and the new stripe written to free space. Once
that is done, the new stripe is unquiesced and used.)
.SS Examples
(Command output shown in examples may change.)
Converting raid6_n_6 to raid6_nr with rotating data/parity.
This conversion naturally follows a previous conversion from striped/raid0
to raid6_n_6 (shown above). It completes the transition to a more
traditional RAID6.
.nf
# lvs -o lv_name,segtype,sync_percent,data_copies
LV Type Cpy%Sync #Cpy
lv raid6_n_6 100.00 3
[lv_rimage_0] linear
[lv_rimage_1] linear
[lv_rimage_2] linear
[lv_rimage_3] linear
[lv_rimage_4] linear
[lv_rimage_5] linear
[lv_rmeta_0] linear
[lv_rmeta_1] linear
[lv_rmeta_2] linear
[lv_rmeta_3] linear
[lv_rmeta_4] linear
[lv_rmeta_5] linear
# lvconvert --type raid6_nr vg/lv
# lvs -a -o lv_name,segtype,sync_percent,data_copies
LV Type Cpy%Sync #Cpy
lv raid6_nr 100.00 3
[lv_rimage_0] linear
[lv_rimage_0] linear
[lv_rimage_1] linear
[lv_rimage_1] linear
[lv_rimage_2] linear
[lv_rimage_2] linear
[lv_rimage_3] linear
[lv_rimage_3] linear
[lv_rimage_4] linear
[lv_rimage_5] linear
[lv_rmeta_0] linear
[lv_rmeta_1] linear
[lv_rmeta_2] linear
[lv_rmeta_3] linear
[lv_rmeta_4] linear
[lv_rmeta_5] linear
.fi
The DataLVs are larger (additional segment in each) which provides space
for out-of-place reshaping. The result is:
.nf
# lvs -a -o lv_name,segtype,seg_pe_ranges,dataoffset
LV Type PE Ranges DOff
lv raid6_nr lv_rimage_0:0-32 \\
lv_rimage_1:0-32 \\
lv_rimage_2:0-32 \\
lv_rimage_3:0-32
[lv_rimage_0] linear /dev/sda:0-31 2048
[lv_rimage_0] linear /dev/sda:33-33
[lv_rimage_1] linear /dev/sdaa:0-31 2048
[lv_rimage_1] linear /dev/sdaa:33-33
[lv_rimage_2] linear /dev/sdab:1-33 2048
[lv_rimage_3] linear /dev/sdac:1-33 2048
[lv_rmeta_0] linear /dev/sda:32-32
[lv_rmeta_1] linear /dev/sdaa:32-32
[lv_rmeta_2] linear /dev/sdab:0-0
[lv_rmeta_3] linear /dev/sdac:0-0
.fi
All segments with PE ranges '33-33' provide the out-of-place reshape space.
The dataoffset column shows that the data was moved from initial offset 0 to
2048 sectors on each component DataLV.
For performance reasons the raid6_nr RaidLV can be restriped.
Convert it from 3-way striped to 5-way-striped.
.nf
# lvconvert --stripes 5 vg/lv
Using default stripesize 64.00 KiB.
WARNING: Adding stripes to active logical volume vg/lv will \\
grow it from 99 to 165 extents!
Run "lvresize -l99 vg/lv" to shrink it or use the additional \\
capacity.
Logical volume vg/lv successfully converted.
# lvs vg/lv
LV VG Attr LSize Cpy%Sync
lv vg rwi-a-r-s- 652.00m 52.94
# lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
LV Attr Type PE Ranges DOff
lv rwi-a-r--- raid6_nr lv_rimage_0:0-33 \\
lv_rimage_1:0-33 \\
lv_rimage_2:0-33 ... \\
lv_rimage_5:0-33 \\
lv_rimage_6:0-33 0
[lv_rimage_0] iwi-aor--- linear /dev/sda:0-32 0
[lv_rimage_0] iwi-aor--- linear /dev/sda:34-34
[lv_rimage_1] iwi-aor--- linear /dev/sdaa:0-32 0
[lv_rimage_1] iwi-aor--- linear /dev/sdaa:34-34
[lv_rimage_2] iwi-aor--- linear /dev/sdab:0-32 0
[lv_rimage_2] iwi-aor--- linear /dev/sdab:34-34
[lv_rimage_3] iwi-aor--- linear /dev/sdac:1-34 0
[lv_rimage_4] iwi-aor--- linear /dev/sdad:1-34 0
[lv_rimage_5] iwi-aor--- linear /dev/sdae:1-34 0
[lv_rimage_6] iwi-aor--- linear /dev/sdaf:1-34 0
[lv_rmeta_0] ewi-aor--- linear /dev/sda:33-33
[lv_rmeta_1] ewi-aor--- linear /dev/sdaa:33-33
[lv_rmeta_2] ewi-aor--- linear /dev/sdab:33-33
[lv_rmeta_3] ewi-aor--- linear /dev/sdac:0-0
[lv_rmeta_4] ewi-aor--- linear /dev/sdad:0-0
[lv_rmeta_5] ewi-aor--- linear /dev/sdae:0-0
[lv_rmeta_6] ewi-aor--- linear /dev/sdaf:0-0
.fi
Stripes also can be removed from raid5 and 6.
Convert the 5-way striped raid6_nr LV to 4-way-striped.
The force option needs to be used, because removing stripes
(i.e. image SubLVs) from a RaidLV will shrink its size.
.nf
# lvconvert --stripes 4 vg/lv
Using default stripesize 64.00 KiB.
WARNING: Removing stripes from active logical volume vg/lv will \\
shrink it from 660.00 MiB to 528.00 MiB!
THIS MAY DESTROY (PARTS OF) YOUR DATA!
If that leaves the logical volume larger than 206 extents due \\
to stripe rounding,
you may want to grow the content afterwards (filesystem etc.)
WARNING: to remove freed stripes after the conversion has finished,\\
you have to run "lvconvert --stripes 4 vg/lv"
Logical volume vg/lv successfully converted.
# lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
LV Attr Type PE Ranges DOff
lv rwi-a-r-s- raid6_nr lv_rimage_0:0-33 \\
lv_rimage_1:0-33 \\
lv_rimage_2:0-33 ... \\
lv_rimage_5:0-33 \\
lv_rimage_6:0-33 0
[lv_rimage_0] Iwi-aor--- linear /dev/sda:0-32 0
[lv_rimage_0] Iwi-aor--- linear /dev/sda:34-34
[lv_rimage_1] Iwi-aor--- linear /dev/sdaa:0-32 0
[lv_rimage_1] Iwi-aor--- linear /dev/sdaa:34-34
[lv_rimage_2] Iwi-aor--- linear /dev/sdab:0-32 0
[lv_rimage_2] Iwi-aor--- linear /dev/sdab:34-34
[lv_rimage_3] Iwi-aor--- linear /dev/sdac:1-34 0
[lv_rimage_4] Iwi-aor--- linear /dev/sdad:1-34 0
[lv_rimage_5] Iwi-aor--- linear /dev/sdae:1-34 0
[lv_rimage_6] Iwi-aor-R- linear /dev/sdaf:1-34 0
[lv_rmeta_0] ewi-aor--- linear /dev/sda:33-33
[lv_rmeta_1] ewi-aor--- linear /dev/sdaa:33-33
[lv_rmeta_2] ewi-aor--- linear /dev/sdab:33-33
[lv_rmeta_3] ewi-aor--- linear /dev/sdac:0-0
[lv_rmeta_4] ewi-aor--- linear /dev/sdad:0-0
[lv_rmeta_5] ewi-aor--- linear /dev/sdae:0-0
[lv_rmeta_6] ewi-aor-R- linear /dev/sdaf:0-0
.fi
The 's' in column 9 of the attribute field shows the RaidLV is still reshaping.
The 'R' in the same column of the attribute field shows the freed image Sub LVs which will need removing once the reshaping finished.
.nf
# lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
LV Attr Type PE Ranges DOff
lv rwi-a-r-R- raid6_nr lv_rimage_0:0-33 \\
lv_rimage_1:0-33 \\
lv_rimage_2:0-33 ... \\
lv_rimage_5:0-33 \\
lv_rimage_6:0-33 8192
.fi
Now that the reshape is finished the 'R' atribute on the RaidLV shows images can be removed.
.nf
# lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
LV Attr Type PE Ranges DOff
lv rwi-a-r-R- raid6_nr lv_rimage_0:0-33 \\
lv_rimage_1:0-33 \\
lv_rimage_2:0-33 ... \\
lv_rimage_5:0-33 \\
lv_rimage_6:0-33 8192
.fi
This is achieved by repeating the command ("lvconvert --stripes 4 vg/lv" would be sufficient).
.nf
# lvconvert --stripes 4 vg/lv
Using default stripesize 64.00 KiB.
Logical volume vg/lv successfully converted.
# lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
LV Attr Type PE Ranges DOff
lv rwi-a-r--- raid6_nr lv_rimage_0:0-33 \\
lv_rimage_1:0-33 \\
lv_rimage_2:0-33 ... \\
lv_rimage_5:0-33 8192
[lv_rimage_0] iwi-aor--- linear /dev/sda:0-32 8192
[lv_rimage_0] iwi-aor--- linear /dev/sda:34-34
[lv_rimage_1] iwi-aor--- linear /dev/sdaa:0-32 8192
[lv_rimage_1] iwi-aor--- linear /dev/sdaa:34-34
[lv_rimage_2] iwi-aor--- linear /dev/sdab:0-32 8192
[lv_rimage_2] iwi-aor--- linear /dev/sdab:34-34
[lv_rimage_3] iwi-aor--- linear /dev/sdac:1-34 8192
[lv_rimage_4] iwi-aor--- linear /dev/sdad:1-34 8192
[lv_rimage_5] iwi-aor--- linear /dev/sdae:1-34 8192
[lv_rmeta_0] ewi-aor--- linear /dev/sda:33-33
[lv_rmeta_1] ewi-aor--- linear /dev/sdaa:33-33
[lv_rmeta_2] ewi-aor--- linear /dev/sdab:33-33
[lv_rmeta_3] ewi-aor--- linear /dev/sdac:0-0
[lv_rmeta_4] ewi-aor--- linear /dev/sdad:0-0
[lv_rmeta_5] ewi-aor--- linear /dev/sdae:0-0
# lvs -a -o lv_name,attr,segtype,reshapelen vg
LV Attr Type RSize
lv rwi-a-r--- raid6_nr 24.00m
[lv_rimage_0] iwi-aor--- linear 4.00m
[lv_rimage_0] iwi-aor--- linear
[lv_rimage_1] iwi-aor--- linear 4.00m
[lv_rimage_1] iwi-aor--- linear
[lv_rimage_2] iwi-aor--- linear 4.00m
[lv_rimage_2] iwi-aor--- linear
[lv_rimage_3] iwi-aor--- linear 4.00m
[lv_rimage_4] iwi-aor--- linear 4.00m
[lv_rimage_5] iwi-aor--- linear 4.00m
[lv_rmeta_0] ewi-aor--- linear
[lv_rmeta_1] ewi-aor--- linear
[lv_rmeta_2] ewi-aor--- linear
[lv_rmeta_3] ewi-aor--- linear
[lv_rmeta_4] ewi-aor--- linear
[lv_rmeta_5] ewi-aor--- linear
.fi
Future developments might include automatic removal of the freed images.
If the reshape space shall be removed any lvconvert command not changing the layout can be used:
.nf
# lvconvert --stripes 4 vg/lv
Using default stripesize 64.00 KiB.
No change in RAID LV vg/lv layout, freeing reshape space.
Logical volume vg/lv successfully converted.
# lvs -a -o lv_name,attr,segtype,reshapelen vg
LV Attr Type RSize
lv rwi-a-r--- raid6_nr 0
[lv_rimage_0] iwi-aor--- linear 0
[lv_rimage_0] iwi-aor--- linear
[lv_rimage_1] iwi-aor--- linear 0
[lv_rimage_1] iwi-aor--- linear
[lv_rimage_2] iwi-aor--- linear 0
[lv_rimage_2] iwi-aor--- linear
[lv_rimage_3] iwi-aor--- linear 0
[lv_rimage_4] iwi-aor--- linear 0
[lv_rimage_5] iwi-aor--- linear 0
[lv_rmeta_0] ewi-aor--- linear
[lv_rmeta_1] ewi-aor--- linear
[lv_rmeta_2] ewi-aor--- linear
[lv_rmeta_3] ewi-aor--- linear
[lv_rmeta_4] ewi-aor--- linear
[lv_rmeta_5] ewi-aor--- linear
.fi
In case the RaidLV should be converted to striped:
.nf
# lvconvert --type striped vg/lv
Unable to convert LV vg/lv from raid6_nr to striped.
Converting vg/lv from raid6_nr is directly possible to the \\
following layouts:
raid6_nc
raid6_zr
raid6_la_6
raid6_ls_6
raid6_ra_6
raid6_rs_6
raid6_n_6
.fi
A direct conversion isn't possible thus the command informed about the possible ones.
raid6_n_6 is suitable to convert to striped so convert to it first (this is a reshape
changing the raid6 layout from raid6_nr to raid6_n_6).
.nf
# lvconvert --type raid6_n_6
Using default stripesize 64.00 KiB.
Converting raid6_nr LV vg/lv to raid6_n_6.
Are you sure you want to convert raid6_nr LV vg/lv? [y/n]: y
Logical volume vg/lv successfully converted.
.fi
Wait for the reshape to finish.
.nf
# lvconvert --type striped vg/lv
Logical volume vg/lv successfully converted.
# lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
LV Attr Type PE Ranges DOff
lv -wi-a----- striped /dev/sda:2-32 \\
/dev/sdaa:2-32 \\
/dev/sdab:2-32 \\
/dev/sdac:3-33
lv -wi-a----- striped /dev/sda:34-35 \\
/dev/sdaa:34-35 \\
/dev/sdab:34-35 \\
/dev/sdac:34-35
.fi
From striped we can convert to raid10
.nf
# lvconvert --type raid10 vg/lv
Using default stripesize 64.00 KiB.
Logical volume vg/lv successfully converted.
# lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
LV Attr Type PE Ranges DOff
lv rwi-a-r--- raid10 lv_rimage_0:0-32 \\
lv_rimage_4:0-32 \\
lv_rimage_1:0-32 ... \\
lv_rimage_3:0-32 \\
lv_rimage_7:0-32 0
# lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
WARNING: Cannot find matching striped segment for vg/lv_rimage_3.
LV Attr Type PE Ranges DOff
lv rwi-a-r--- raid10 lv_rimage_0:0-32 \\
lv_rimage_4:0-32 \\
lv_rimage_1:0-32 ... \\
lv_rimage_3:0-32 \\
lv_rimage_7:0-32 0
[lv_rimage_0] iwi-aor--- linear /dev/sda:2-32 0
[lv_rimage_0] iwi-aor--- linear /dev/sda:34-35
[lv_rimage_1] iwi-aor--- linear /dev/sdaa:2-32 0
[lv_rimage_1] iwi-aor--- linear /dev/sdaa:34-35
[lv_rimage_2] iwi-aor--- linear /dev/sdab:2-32 0
[lv_rimage_2] iwi-aor--- linear /dev/sdab:34-35
[lv_rimage_3] iwi-XXr--- linear /dev/sdac:3-35 0
[lv_rimage_4] iwi-aor--- linear /dev/sdad:1-33 0
[lv_rimage_5] iwi-aor--- linear /dev/sdae:1-33 0
[lv_rimage_6] iwi-aor--- linear /dev/sdaf:1-33 0
[lv_rimage_7] iwi-aor--- linear /dev/sdag:1-33 0
[lv_rmeta_0] ewi-aor--- linear /dev/sda:0-0
[lv_rmeta_1] ewi-aor--- linear /dev/sdaa:0-0
[lv_rmeta_2] ewi-aor--- linear /dev/sdab:0-0
[lv_rmeta_3] ewi-aor--- linear /dev/sdac:0-0
[lv_rmeta_4] ewi-aor--- linear /dev/sdad:0-0
[lv_rmeta_5] ewi-aor--- linear /dev/sdae:0-0
[lv_rmeta_6] ewi-aor--- linear /dev/sdaf:0-0
[lv_rmeta_7] ewi-aor--- linear /dev/sdag:0-0
.fi
raid10 allows to add stripes but can't remove them.
A more elaborate example to convert from linear to striped
with interim conversions to raid1 then raid5 followed
by restripe (4 steps).
We start with the linear LV.
.nf
# lvs -a -o name,size,segtype,syncpercent,datastripes,\\
stripesize,reshapelenle,devices vg
LV LSize Type Cpy%Sync #DStr Stripe RSize Devices
lv 128.00m linear 1 0 /dev/sda(0)
.fi
Then convert it to a 2-way raid1.
.nf
# lvconvert --mirrors 1 vg/lv
Logical volume vg/lv successfully converted.
# lvs -a -o name,size,segtype,datastripes,\\
stripesize,reshapelenle,devices vg
LV LSize Type #DStr Stripe RSize Devices
lv 128.00m raid1 2 0 lv_rimage_0(0),\\
lv_rimage_1(0)
[lv_rimage_0] 128.00m linear 1 0 /dev/sda(0)
[lv_rimage_1] 128.00m linear 1 0 /dev/sdhx(1)
[lv_rmeta_0] 4.00m linear 1 0 /dev/sda(32)
[lv_rmeta_1] 4.00m linear 1 0 /dev/sdhx(0)
.fi
Once the raid1 LV is fully synchronized we convert it to raid5_n (only 2-way raid1
LVs can be converted to raid5). We select raid5_n here because it has dedicated parity
SubLVs at the end and can be converted to striped directly without any additional
conversion.
.nf
# lvconvert --type raid5_n vg/lv
Using default stripesize 64.00 KiB.
Logical volume vg/lv successfully converted.
# lvs -a -o name,size,segtype,syncpercent,datastripes,\\
stripesize,reshapelenle,devices vg
LV LSize Type #DStr Stripe RSize Devices
lv 128.00m raid5_n 1 64.00k 0 lv_rimage_0(0),\\
lv_rimage_1(0)
[lv_rimage_0] 128.00m linear 1 0 0 /dev/sda(0)
[lv_rimage_1] 128.00m linear 1 0 0 /dev/sdhx(1)
[lv_rmeta_0] 4.00m linear 1 0 /dev/sda(32)
[lv_rmeta_1] 4.00m linear 1 0 /dev/sdhx(0)
.fi
Now we'll change the number of data stripes from 1 to 5 and request 128K stripe size
in one command. This will grow the size of the LV by a factor of 5 (we add 4 data stripes
to the one given). That additonal space can be used by e.g. growing any contained filesystem
or the LV can be reduced in size after the reshaping conversion has finished.
.nf
# lvconvert --stripesize 128k --stripes 5 vg/lv
Converting stripesize 64.00 KiB of raid5_n LV vg/lv to 128.00 KiB.
WARNING: Adding stripes to active logical volume vg/lv will grow \\
it from 32 to 160 extents!
Run "lvresize -l32 vg/lv" to shrink it or use the additional capacity.
Logical volume vg/lv successfully converted.
# lvs -a -o name,size,segtype,datastripes,\\
stripesize,reshapelenle,devices
LV LSize Type #DStr Stripe RSize Devices
lv 640.00m raid5_n 5 128.00k 6 lv_rimage_0(0),\\
lv_rimage_1(0),\\
lv_rimage_2(0),\\
lv_rimage_3(0),\\
lv_rimage_4(0),\\
lv_rimage_5(0)
[lv_rimage_0] 132.00m linear 1 0 1 /dev/sda(33)
[lv_rimage_0] 132.00m linear 1 0 /dev/sda(0)
[lv_rimage_1] 132.00m linear 1 0 1 /dev/sdhx(33)
[lv_rimage_1] 132.00m linear 1 0 /dev/sdhx(1)
[lv_rimage_2] 132.00m linear 1 0 1 /dev/sdhw(33)
[lv_rimage_2] 132.00m linear 1 0 /dev/sdhw(1)
[lv_rimage_3] 132.00m linear 1 0 1 /dev/sdhv(33)
[lv_rimage_3] 132.00m linear 1 0 /dev/sdhv(1)
[lv_rimage_4] 132.00m linear 1 0 1 /dev/sdhu(33)
[lv_rimage_4] 132.00m linear 1 0 /dev/sdhu(1)
[lv_rimage_5] 132.00m linear 1 0 1 /dev/sdht(33)
[lv_rimage_5] 132.00m linear 1 0 /dev/sdht(1)
[lv_rmeta_0] 4.00m linear 1 0 /dev/sda(32)
[lv_rmeta_1] 4.00m linear 1 0 /dev/sdhx(0)
[lv_rmeta_2] 4.00m linear 1 0 /dev/sdhw(0)
[lv_rmeta_3] 4.00m linear 1 0 /dev/sdhv(0)
[lv_rmeta_4] 4.00m linear 1 0 /dev/sdhu(0)
[lv_rmeta_5] 4.00m linear 1 0 /dev/sdht(0)
.fi
Once the conversion has finished we can can convert to striped.
.nf
# lvconvert --type striped vg/lv
Logical volume vg/lv successfully converted.
# lvs -a -o name,size,segtype,datastripes,\\
stripesize,reshapelenle,devices vg
LV LSize Type #DStr Stripe RSize Devices
lv 640.00m striped 5 128.00k /dev/sda(33),\\
/dev/sdhx(33),\\
/dev/sdhw(33),\\
/dev/sdhv(33),\\
/dev/sdhu(33)
lv 640.00m striped 5 128.00k /dev/sda(0),\\
/dev/sdhx(1),\\
/dev/sdhw(1),\\
/dev/sdhv(1),\\
/dev/sdhu(1)
.fi
Reversing these steps will convert a given striped LV to linear.
Mind the facts that stripes are removed thus the capacity of the RaidLV will shrink
and that changing the RaidLV layout will influence its performance.
"lvconvert --stripes 1 vg/lv" for converting to 1 stripe will inform upfront about
the reduced size to allow for resizing the content or growing the RaidLV before
actually converting to 1 stripe. The \fB--force\fP option is needed to
allow stripe removing conversions to prevent data loss.
Of course any interim step can be the intended last one (e.g. striped -> raid1).
..
.SH RAID5 Variants
raid5_ls
.br
\[bu]
RAID5 left symmetric
.br
\[bu]
Rotating parity N with data restart
raid5_la
.br
\[bu]
RAID5 left symmetric
.br
\[bu]
Rotating parity N with data continuation
raid5_rs
.br
\[bu]
RAID5 right symmetric
.br
\[bu]
Rotating parity 0 with data restart
raid5_ra
.br
\[bu]
RAID5 right asymmetric
.br
\[bu]
Rotating parity 0 with data continuation
raid5_n
.br
\[bu]
RAID5 parity n
.br
\[bu]
Dedicated parity device n used for striped/raid0 conversions
.br
\[bu]
Used for RAID Takeover
.SH RAID6 Variants
raid6
.br
\[bu]
RAID6 zero restart (aka left symmetric)
.br
\[bu]
Rotating parity 0 with data restart
.br
\[bu]
Same as raid6_zr
raid6_zr
.br
\[bu]
RAID6 zero restart (aka left symmetric)
.br
\[bu]
Rotating parity 0 with data restart
raid6_nr
.br
\[bu]
RAID6 N restart (aka right symmetric)
.br
\[bu]
Rotating parity N with data restart
raid6_nc
.br
\[bu]
RAID6 N continue
.br
\[bu]
Rotating parity N with data continuation
raid6_n_6
.br
\[bu]
RAID6 last parity devices
.br
\[bu]
Fixed dedicated last devices (P-Syndrome N-1 and Q-Syndrome N)
.RS 2
with striped data used for striped/raid0 conversions
.RE
.br
\[bu]
Used for RAID Takeover
raid6_{ls,rs,la,ra}_6
.br
\[bu]
RAID6 last parity device
.br
\[bu]
Dedicated last parity device used for conversions from/to
.RS 2
raid5_{ls,rs,la,ra}
.RE
raid6_ls_6
.br
\[bu]
RAID6 N continue
.br
\[bu]
Same as raid5_ls for N-1 devices with fixed Q-Syndrome N
.br
\[bu]
Used for RAID Takeover
raid6_la_6
.br
\[bu]
RAID6 N continue
.br
\[bu]
Same as raid5_la for N-1 devices with fixed Q-Syndrome N
.br
\[bu]
Used forRAID Takeover
raid6_rs_6
.br
\[bu]
RAID6 N continue
.br
\[bu]
Same as raid5_rs for N-1 devices with fixed Q-Syndrome N
.br
\[bu]
Used for RAID Takeover
raid6_ra_6
.br
\[bu]
RAID6 N continue
.br
\[bu]
ame as raid5_ra for N-1 devices with fixed Q-Syndrome N
.br
\[bu]
Used for RAID Takeover
.ig
.SH RAID Duplication
RAID LV conversion (takeover or reshaping) can be done out-of-place by
copying the LV data onto new devices while changing the RAID properties.
Copying avoids modifying the original LV but requires additional devices.
Once the LV data has been copied/converted onto the new devices, there are
multiple options:
1. The RAID LV can be switched over to run from just the new devices, and
the original copy of the data removed. The converted LV then has the new
RAID properties, and exists on new devices. The old devices holding the
original data can be removed or reused.
2. The new copy of the data can be dropped, leaving the original RAID LV
unchanged and using its original devices.
3. The new copy of the data can be separated and used as a new independent
LV, leaving the original RAID LV unchanged on its original devices.
The command to start duplication is:
.B lvconvert --type
.I RaidLevel
[\fB--stripes\fP \fINumber\fP \fB--stripesize\fP \fISize\fP]
.RS
.B --duplicate
.I LV
[\fIPVs\fP]
.RE
.HP
.B --duplicate
.br
Specifies that the LV conversion should be done out-of-place, copying
LV data to new devices while converting.
.HP
.BR --type , --stripes , --stripesize
.br
Specifies the RAID properties to use when creating the copy.
.P
\fIPVs\fP specifies the new devices to use.
The steps in the duplication process:
.IP \(bu 3
LVM creates a new LV on new devices using the specified RAID properties
(type, stripes, etc) and optionally specified devices.
.IP \(bu 3
LVM changes the visible RAID LV to type raid1, making the original LV the
first raid1 image (SubLV 0), and the new LV the second raid1 image
(SubLV 1).
.IP \(bu 3
The RAID1 synchronization process copies data from the original LV
image (SubLV 0) to the new LV image (SubLV 1).
.IP \(bu 3
When synchronization is complete, the original and new LVs are
mirror images of each other and can be separated.
.P
The duplication process retains both the original and new LVs (both
SubLVs) until an explicit unduplicate command is run to separate them. The
unduplicate command specifies if the original LV should use the old
devices (SubLV 0) or the new devices (SubLV 1).
To make the RAID LV use the data on the old devices, and drop the copy on
the new devices, specify the name of SubLV 0 (suffix _dup_0):
.B lvconvert --unduplicate
.BI --name
.IB LV _dup_0
.I LV
To make the RAID LV use the data copy on the new devices, and drop the old
devices, specify the name of SubLV 1 (suffix _dup_1):
.B lvconvert --unduplicate
.BI --name
.IB LV _dup_1
.I LV
FIXME: To make the LV use the data on the original devices, but keep the
data copy as a new LV, ...
FIXME: include how splitmirrors can be used.
.SH RAID1E
TODO
..
.SH History
The 2.6.38-rc1 version of the Linux kernel introduced a device-mapper
target to interface with the software RAID (MD) personalities. This
provided device-mapper with RAID 4/5/6 capabilities and a larger
development community. Later, support for RAID1, RAID10, and RAID1E (RAID
10 variants) were added. Support for these new kernel RAID targets was
added to LVM version 2.02.87. The capabilities of the LVM \fBraid1\fP
type have surpassed the old \fBmirror\fP type. raid1 is now recommended
instead of mirror. raid1 became the default for mirroring in LVM version
2.02.100.
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