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man: expanded explanation of pvscan
Related to it's role with lvmetad and auto-activation.
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David Teigland
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146
man/pvscan.8.in
146
man/pvscan.8.in
@ -1,6 +1,7 @@
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.TH PVSCAN 8 "LVM TOOLS #VERSION#" "Sistina Software UK" \" -*- nroff -*-
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.SH NAME
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pvscan \(em scan all disks for physical volumes
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.SH SYNOPSIS
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.B pvscan
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.RB [ \-\-commandprofile
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@ -30,9 +31,118 @@ pvscan \(em scan all disks for physical volumes
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.IR DevicePath
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.IR major:minor ]...
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.SH DESCRIPTION
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pvscan scans all supported LVM block devices in the system for
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physical volumes.
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pvscan scans all supported LVM block devices in the system for physical
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volumes.
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.SS Scanning with lvmetad
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pvscan operates differently when used with the
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.BR lvmetad (8)
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daemon.
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Scanning disks is required to read LVM metadata and identify LVM PVs.
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Once read, lvmetad caches the metadata so that LVM commands can read it
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without repeatedly scanning disks. This is helpful because scanning disks
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is time consuming, and frequent scanning may interfere with the normal
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work of the system and disks.
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When lvmetad is not used, LVM commands revert to scanning disks to read
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metadata. Any LVM command that needs metadata will scan disks for it;
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running the pvscan command is not necessary for the sake of other LVM
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commands.
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When lvmetad is used, LVM commands avoid scanning disks by reading
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metadata from lvmetad. When new disks appear, they must be scanned so
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their metadata can be cached in lvmetad. This is done by the command
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pvscan \-\-cache, which scans disks and passes the metadata to lvmetad.
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The pvscan \-\-cache command is typically run automatically by system
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services when a new device appears. Users do not generally need to run
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this command if the system and lvmetad are running properly.
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Many scripts contain unnecessary pvscan (or vgscan) commands for
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historical reasons. To avoid disrupting the system with extraneous disk
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scanning, an ordinary pvscan (without \-\-cache) will simply read metadata
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from lvmetad like other LVM commands. It does not do anything beyond
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displaying the current state of the cache.
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.I Notes
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.IP \[bu] 2
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When given specific device name arguments, pvscan \-\-cache will only
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read the named devices.
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.IP \[bu] 2
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LVM udev rules and systemd services are used to intiate automatic device
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scanning.
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.IP \[bu] 2
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To prevent devices from being scanned by pvscan --cache, add them
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to
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.BR lvm.conf (5)
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.B devices/global_filter.
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The devices/filter setting does not
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apply to system level scanning.
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For more information, see:
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.br
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.B lvmconfig --withcomments devices/global_filter
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.IP \[bu] 2
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If lvmetad is started or restarted after devices are visible, or
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if the global_filter has changed, then all devices must be rescanned
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for metadata with the command pvscan \-\-cache.
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.IP \[bu] 2
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lvmetad ignores older metadata formats, e.g. lvm1, and should not be
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used if they exist.
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.IP \[bu] 2
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To notify lvmetad about a device that is no longer present, the major and
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minor numbers must be given, not the path.
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.SS Automatic activation
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When event-driven system services detect a new LVM device, the first step
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is to automatically scan and cache the metadata from the device. This is
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done by pvscan \-\-cache. A second step is to automatically activate LVs
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that are present on the new device. This auto-activation is done by the
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same pvscan \-\-cache command when the option '\-a|\-\-activate ay' is
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included.
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Auto-activation of VGs or LVs can be enabled/disabled using:
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.br
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.BR lvm.conf (5)
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.B activation/auto_activation_volume_list
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For more information, see:
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.br
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.B lvmconfig --withcomments activation/auto_activation_volume_list
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When this setting is undefined, all LVs are auto-activated (when lvm is
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fully integrated with the event-driven system services.)
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When a VG or LV is not auto-activated, traditional activation using
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vgchange or lvchange -a|--activate is needed.
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.I Notes
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.IP \[bu] 2
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pvscan auto-activation can be only done in combination with \-\-cache.
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.IP \[bu] 2
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Auto-activation is designated by the "a" argument in '-a|--activate ay'.
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This is meant to distinguish system generated commands from explicit user
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commands, although it can be used in any activation command. Whenever it
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is used, the auto_activation_volume_list is applied.
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.IP \[bu] 2
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Auto-activation is not yet supported for LVs that are part of partial or
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clustered volume groups.
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.P
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.SH OPTIONS
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See \fBlvm\fP(8) for common options.
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.TP
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@ -46,37 +156,19 @@ Only show physical volumes not belonging to any volume group.
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Short listing format.
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.TP
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.BR \-u ", " \-\-uuid
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Show UUIDs (Uniform Unique Identifiers) in addition to device special names.
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Show UUIDs in addition to device names.
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.TP
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.BR \-a ", " \-\-activate " " \fIay
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Together with the information already cached in lvmetad, automatically activate
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any logical volumes that become activatable after the scan done on one or more devices.
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The logical volume to autoactivate is matched against the
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activation/auto_activation_volume_list set in lvm.conf. If this list is not set, then
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all volumes are considered for autoactivation. The autoactivation is not yet
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supported for logical volumes that are part of partial or clustered volume groups.
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Automatically activate any logical volumes that are possible to activate
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with the addition of the new devices.
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.TP
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.BR \-b ", " \-\-background
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Run the command in the background.
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.TP
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.BR \-\-cache " [" \-\-major " " \fImajor " " \-\-minor " " \fIminor " | " \fIDevicePath " | " \fImajor:minor " ]..."
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Scan one or more devices and instruct the lvmetad daemon to update its cached
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state accordingly. Called internally by udev rules.
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All devices listed explicitly are processed \fBregardless\fP of any device
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filters set using \fBdevices/filter\fP configuration setting. To filter
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devices even in this case, the \fBdevices/global_filter\fP must be used.
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If lvmetad has not yet cached any metadata or the filters have recently been
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changed, then all devices may be scanned, effectively ignoring the rest of
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the command line. Otherwise, if all the devices referenced on the command line
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contain metadata in the default lvm2 format, other devices are not accessed.
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If metadata written using the obsolete GFS pool or lvm1 format is encountered,
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this is ignored and so lvmetad should not be used.
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To notify lvmetad about a device that is not present in the system anymore,
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\fB\-\-major\fP and \fB\-\-minor\fP pair of that device must be always supplied,
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not \fBDevicePath\fP as pvscan is not able to translate the \fBDevicePath\fP
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into major and minor pair which lvmetad requires.
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Scan one or more devices and send the metadata to lvmetad.
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.SH SEE ALSO
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.BR lvm (8),
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.BR lvmetad (8),
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.BR pvcreate (8),
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.BR pvdisplay (8)
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.BR lvmconfig (8),
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.BR lvmetad (8)
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