Saturday, February 16, 2008

What is Hot Spare

What is Hot Spare
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 physicalvolume missing due to write failures. It then starts the migration of data to the hot sparedisk.Minimum hot spare requirementsThe following is a list of minimal hot sparing requirements enforced by the operatingsystem.- 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 alreadyin the volume group. Good practice dictates having enough hot spares tocover your largest mirrored disk.Hot spare policyThe chpv and the chvg commands are enhanced with a new -h argument. This allows youto designate disks as hot spares in a volume group and to specify a policy to be used in thecase of failing disks.The following four values are valid for the hot spare policy argument (-h):Synchronization policyThere is a new -s argument for the chvg command that is used to specify synchronizationcharacteristics.The following two values are valid for the synchronization argument (-s):ExamplesThe following command marks hdisk1 as a hot spare disk:# chpv -hy hdisk1The following command sets an automatic migration policy which uses the smallest hotspare that is large enough to replace the failing disk, and automatically tries to synchronizestale partitions:# chvg -hy -sy testvgArgument Descriptiony (lower case)Automatically migrates partitions from one failing disk to one sparedisk. From the pool of hot spare disks, the smallest one which is bigenough to substitute for the failing disk will be used.Y (upper case)Automatically migrates partitions from a failing disk, but might usethe complete pool of hot spare disks.nNo automatic migration will take place. This is the default value for avolume group.rRemoves all disks from the pool of hot spare disks for this volume

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