12-06-2009
When using SCSI devices, the path is clearly defined based on the SCSI ID of the controller, the port that the device is connected to, etc.
SATA/SAS drives do not have this luxury, as they are not defined on a chain. This causes some confusion, as when /dev/sda comes up next time, it may be /dev/sdc.
dev-mapper uses a different, non-volatile ID such as UUID or disk label (contained in metadata) to keep disk paths consistent, through its abstraction.
Basically, the short version is:
To make sure that your devices are named and in a consistent path, dev-mapper handles the devices so that your / is found properly each bootup and is not confused with /tmp or some other file system on another device.
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VGREDUCE(8) System Manager's Manual VGREDUCE(8)
NAME
vgreduce - reduce a volume group
SYNOPSIS
vgreduce [-a|--all] [-A|--autobackup y|n] [-d|--debug] [-h|-?|--help] [--removemissing] [-t|--test] [-v|--verbose] VolumeGroupName [Physi-
calVolumePath...]
DESCRIPTION
vgreduce allows you to remove one or more unused physical volumes from a volume group.
OPTIONS
See lvm for common options.
-a, --all
Removes all empty physical volumes if none are given on command line.
--removemissing
Removes all missing physical volumes from the volume group, if there are no logical volumes allocated on those. This resumes normal
operation of the volume group (new logical volumes may again be created, changed and so on).
If this is not possible (there are logical volumes referencing the missing physical volumes) and you cannot or do not want to remove
them manually, you can run this option with --force to have vgreduce remove any partial LVs.
Any logical volumes and dependent snapshots that were partly on the missing disks get removed completely. This includes those parts
that lie on disks that are still present.
If your logical volumes spanned several disks including the ones that are lost, you might want to try to salvage data first by acti-
vating your logical volumes with --partial as described in lvm (8).
SEE ALSO
lvm(8), vgextend(8)
Sistina Software UK LVM TOOLS 2.02.95(2) (2012-03-06) VGREDUCE(8)