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Saturday, November 17, 2007

What is an LVM hot spare?

A hot spare is a disk or group of disks used to replace a failing disk. LVM marks a physical
volume missing due to write failures. It then starts the migration of data to the hot spare
disk.
Minimum hot spare requirements
The following is a list of minimal hot sparing requirements enforced by the operating
system.
- Spares are allocated and used by volume group
- Logical volumes must be mirrored
- All logical partitions on hot spare disks must be unallocated
- Hot spare disks must have at least equal capacity to the smallest disk already
in the volume group. Good practice dictates having enough hot spares to
cover your largest mirrored disk.
Hot spare policy
The chpv and the chvg commands are enhanced with a new -h argument. This allows you
to designate disks as hot spares in a volume group and to specify a policy to be used in the
case of failing disks.
The following four values are valid for the hot spare policy argument (-h):
Synchronization policy
There is a new -s argument for the chvg command that is used to specify synchronization
characteristics.
The following two values are valid for the synchronization argument (-s):
Examples
The following command marks hdisk1 as a hot spare disk:
# chpv -hy hdisk1
The following command sets an automatic migration policy which uses the smallest hot
spare that is large enough to replace the failing disk, and automatically tries to synchronize
stale partitions:
# chvg -hy -sy testvg
Argument Description
y (lower case)
Automatically migrates partitions from one failing disk to one spare
disk. From the pool of hot spare disks, the smallest one which is big
enough to substitute for the failing disk will be used.
Y (upper case)
Automatically migrates partitions from a failing disk, but might use
the complete pool of hot spare disks.
n
No automatic migration will take place. This is the default value for a
volume group.
r
Removes all disks from the pool of hot spare disks for this volume

1 comment:

Vea said...

Good for people to know.