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man: expanded explanation of lvmetad

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David Teigland 2015-05-01 14:45:50 -05:00
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man/lvmetad.8.in
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@ -1,6 +1,7 @@
.TH LVMETAD 8 "LVM TOOLS #VERSION#" "Red Hat Inc" \" -*- nroff -*-
.SH NAME
lvmetad \(em LVM metadata cache daemon
.SH SYNOPSIS
.B lvmetad
.RB [ \-l
@ -13,16 +14,56 @@ lvmetad \(em LVM metadata cache daemon
.RB [ \-h ]
.RB [ \-V ]
.RB [ \-? ]
.SH DESCRIPTION
lvmetad is a metadata caching daemon for LVM. The daemon receives notifications
from udev rules (which must be installed for LVM to work correctly when lvmetad
is in use). Through these notifications, lvmetad has an up-to-date and
consistent image of the volume groups available in the system.
lvmetad is used by LVM only if it is enabled in \fBlvm.conf\fP(5) by specifying the
\fBglobal/use_lvmetad\fP setting. If this is not defined in the LVM configuration
explicitly then default setting is used instead (see the output of
\fBlvmconfig \-\-type default global/use_lvmetad\fP command).
.SH DESCRIPTION
The lvmetad daemon caches LVM metadata, so that LVM commands can read
metadata without scanning disks.
Metadata caching can be an advantage because scanning disks is time
consuming and may interfere with the normal work of the system and disks.
lvmetad does not read metadata from disks itself. The 'pvscan \-\-cache'
command scans disks, reads the LVM metadata and sends it to lvmetad.
New LVM disks that appear on the system must be scanned by pvscan before
lvmetad knows about them. If lvmetad does not know about a disk, then LVM
commands using lvmetad will also not know about it. When disks are added
or removed from the system, lvmetad must be updated.
lvmetad is usually combined with event-based system services that
automatically run pvscan \-\-cache on new disks. This way, the lvmetad
cache is automatically updated with metadata from new disks when they
appear. LVM udev rules and systemd services implement this automation.
Automatic scanning is usually combined with automatic activation. For
more information, see
.BR pvscan (8).
If lvmetad is started or restarted after disks have been added to the
system, or if the global_filter has changed, the cache must be updated by
running 'pvscan \-\-cache'.
When lvmetad is not used, LVM commands revert to scanning disks for LVM
metadata.
Use of lvmetad is enabled/disabled by:
.br
.BR lvm.conf (5)
.B global/use_lvmetad
For more information on this setting, see:
.br
.B lvmconfig \-\-withcomments global/use_lvmetad
To ignore disks from LVM at the system level, e.g. lvmetad, pvscan use:
.br
.BR lvm.conf (5)
.B devices/global_filter
For more information on this setting, see
.br
.B lvmconfig \-\-withcomments devices/global_filter
.SH OPTIONS
To run the daemon in a test environment both the pidfile_path and the
@ -68,4 +109,6 @@ Path for the socket file.
.SH SEE ALSO
.BR lvm (8),
.BR lvm.conf (5)
.BR lvmconfig (8),
.BR lvm.conf (5),
.BR pvscan (8)