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~> lvconvert --splitmirrors 1 --trackchanges vg/lv
The '--trackchanges' option allows a user the ability to use an image of
a RAID1 array for the purposes of temporary read-only access. The image
can be merged back into the array at a later time and only the blocks that
have changed in the array since the split will be resync'ed. This
operation can be thought of as a partial split. The image is never completely
extracted from the array, in that the array reserves the position the device
occupied and tracks the differences between the array and the split image via
a bitmap. The image itself is rendered read-only and the name (<LV>_rimage_*)
cannot be changed. The user can complete the split (permanently splitting the
image from the array) by re-issuing the 'lvconvert' command without the
'--trackchanges' argument and specifying the '--name' argument.
~> lvconvert --splitmirrors 1 --name my_split vg/lv
Merging the tracked image back into the array is done with the '--merge'
option (included in a follow-on patch).
~> lvconvert --merge vg/lv_rimage_<n>
The internal mechanics of this are relatively simple. The 'raid' device-
mapper target allows for the specification of an empty slot in an array
via '- -'. This is what will be used if a partial activation of an array
is ever required. (It would also be possible to use 'error' targets in
place of the '- -'.) If a RAID image is found to be both read-only and
visible, then it is considered separate from the array and '- -' is used
to hold it's position in the array. So, all that needs to be done to
temporarily split an image from the array /and/ cause the kernel target's
bitmap to track (aka "mark") changes made is to make the specified image
visible and read-only. To merge the device back into the array, the image
needs to be returned to the read/write state of the top-level LV and made
invisible.
page.
Add ->target_name to segtype_handler to allow a more specific target
name to be returned based on the state of the segment.
Result of trying to merge a snapshot using a kernel that doesn't have
the snapshot-merge target:
Before:
# lvconvert --merge vg/snap
Can't expand LV lv: snapshot target support missing from kernel?
Failed to suspend origin lv
After:
# lvconvert --merge vg/snap
Can't process LV lv: snapshot-merge target support missing from kernel?
Failed to suspend origin lv
Unable to merge LV "snap" into it's origin.
{core|disk|mirrored}. However, under the options section they are
described as {core|disk} - even though the 'mirrored' argument is
described.
s/'{core|disk}'/'{core|disk|mirrored}'/
This check-in enables the 'mirrored' log type. It can be specified
by using the '--mirrorlog' option as follows:
#> lvcreate -m1 --mirrorlog mirrored -L 5G -n lv vg
I've also included a couple updates to the testsuite. These updates
include tests for the new log type, and some fixes to some of the
*lvconvert* tests.
Switch lvconvert's --merge code over to using process_each_lv(). Doing
so adds support for a single 'lvconvert --merge' to start merging
multiple LVs (which includes @tag expansion).
Add 'lvconvert --merge @tag' testing to test/t-snapshot-merge.sh
Adjust man/lvconvert.8.in to reflect these expanded capabilities.
The lvconvert.c implementation requires rereading the VG each iteration
of process_each_lv(). Otherwise a stale VG instance associated with
the LV passed to lvconvert_single_merge() would result in stale VG
metadata being written back out to disk. This overwrote new metadata
that was written when a previous snapshot LV finished merging (via
lvconvert_poll). This is only an issue when merging multiple LVs that
share the same VG (a single VG is typical for most LVM configurations on
system disks).
In the end this new support is very useful for performing a "system
rollback" that requires multiple snapshot LVs be merged to their
respective origin LV.
The yum-utils 'fs-snapshot' plugin tags all snapshot LVs that it creates
with a common 'snapshot_tag' that is unique to the yum transaction.
Rolling back a yum transaction, that created LVM snapshots with the tag
'yum_20100129133223', is as simple as:
lvconvert --merge @yum_20100129133223
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
It is pretty much the same as reducing the number of
mirror legs, but we just don't delete them afterwards.
The following command line interface is enforced:
prompt> lvconvert --splitmirror <n> -n <name> <VG>/<LV>
where 'n' is the number of images to split off, and
where 'name' is the name of the newly split off logical volume.
If more than one leg is split off, a new mirror will be the
result. The newly split off mirror will have a 'core' log.
Example:
[root@bp-01 LVM2]# !lvs
lvs -a -o name,copy_percent,devices
LV Copy% Devices
lv 100.00 lv_mimage_0(0),lv_mimage_1(0),lv_mimage_2(0),lv_mimage_3(0)
[lv_mimage_0] /dev/sdb1(0)
[lv_mimage_1] /dev/sdc1(0)
[lv_mimage_2] /dev/sdd1(0)
[lv_mimage_3] /dev/sde1(0)
[lv_mlog] /dev/sdi1(0)
[root@bp-01 LVM2]# lvconvert --splitmirrors 2 --name split vg/lv /dev/sd[ce]1
Logical volume lv converted.
[root@bp-01 LVM2]# !lvs
lvs -a -o name,copy_percent,devices
LV Copy% Devices
lv 100.00 lv_mimage_0(0),lv_mimage_2(0)
[lv_mimage_0] /dev/sdb1(0)
[lv_mimage_2] /dev/sdd1(0)
[lv_mlog] /dev/sdi1(0)
split 100.00 split_mimage_0(0),split_mimage_1(0)
[split_mimage_0] /dev/sde1(0)
[split_mimage_1] /dev/sdc1(0)
It can be seen that '--splitmirror <n>' is exactly the same
as '--mirrors -<n>' (note the minus sign), except there is the
additional notion to keep the image being detached from the
mirror instead of just throwing it away.
Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
