Red Hat Bugzilla – Bug 857530
Document lvmetad in 6.4, if deemed necessary for users
Last modified: 2013-02-25 13:30:41 EST
Ok, I'll try to follow up in a couple of weeks about the status of the command (re: tech. preview) and follow these guidelines for documentation if it is out of tech. preview for 6.4.
Passed basic LVM tests with lvmetad/use_lvmetad turned on.
Verifying as SanityOnly.
I'm re-opening this bug, since it is a doc bug and I need to be sure that the documentation is correct (and verified).
Also the bug is assigned to me. Since I didn't have the information for this feature until after the development period I'm exceptioning this bug.
I have a draft of what I have added to the LVM document that I will be sending to Petr for his ok.
I have drafted the following text, presumably would work as a new section, 3.6 Metadata Daemon:
LVM can optionally use a central metadata cache, implemented
through a daemon (lvmetad) and an udev rule. The metadata daemon
has two main purposes: it improves performance of LVM commands
and it allows udev to automatically activate logical volumes or
entire volume groups as they become available to the system.
To take advantage of the daemon, it needs to be started through
the "lvm2-lvmetad" service: use "chkconfig lvm2-lvmetad on" to
start the daemon automatically at boot time, and "service
lvm2-lvmetad start" to start it manually. Moreover, LVM needs to
be configured to make use of the daemon. To this effect, set the
lvm.conf variable "global/use_lvmetad" to 1.
Normally, each LVM command issues a disk scan to find all
relevant physical volumes and to read volume group
metadata. However, if the metadata daemon is running and enabled,
this expensive scan can be skipped: lvmetad scans each device
only once, when it becomes available, via udev rules. Especially
on systems with many disks, this can save significant amount of
I/O and reduce the time required to complete LVM operations.
When a new volume group is made available at runtime (eg. through
hotplug or iSCSI), its logical volumes must be activated in order
to be used. When lvmetad is enabled,
the "activation/auto_activation_volume_list" lvm.conf option can
be used to configure a list of volume groups and/or logical
volumes that should be automatically activated. Without lvmetad,
a manual activation is necessary.