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man: include info about disabled lvmetad

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David Teigland 2016-08-03 15:39:43 -05:00
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commit 2b01dca28a
1 changed files with 24 additions and 14 deletions
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38
man/lvmetad.8.in
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@ -19,35 +19,45 @@ lvmetad \(em LVM metadata cache daemon
.SH DESCRIPTION
The lvmetad daemon caches LVM metadata, so that LVM commands can read
metadata without scanning disks.
The lvmetad daemon caches LVM metadata so that LVM commands can read
metadata from the cache rather than scanning disks. This can be an
advantage because scanning disks is time consuming and may interfere with
the normal work of the system. lvmetad can be a disadvantage when disk
event notifications from the system are unreliable.
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. Instead, it relies on
an LVM command, like pvscan \-\-cache, to read metadata from disks and
send it to lvmetad to be cached.
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
New LVM disks that appear on the system must be scanned 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
lvmetad is usually combined with event\-based system services that
automatically run pvscan \-\-cache on disks added or removed. This way,
the 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'.
system, or if the global_filter has changed, the cache must be updated.
This can be done by running pvscan \-\-cache, or it will be done
automatically by the next LVM command that's run.
When lvmetad is not used, LVM commands revert to scanning disks for LVM
metadata.
In some cases, lvmetad will be temporarily disabled while it continues
running. In this state, LVM commands will ignore the lvmetad cache and
revert to scanning disks. A warning will also be printed which includes
the reason why lvmetad is not being used. The most common reason is the
existence of duplicate PVs (lvmetad cannot cache data for duplicate PVs.)
Once duplicates have been resolved, the lvmetad cache is can be updated
with pvscan \-\-cache and commands will return to using the cache.
Use of lvmetad is enabled/disabled by:
.br
.BR lvm.conf (5)