Dell PowerVault MD1120 Optimization Manual page 18

A REFERENCE GUIDE FOR OPTIMIZING DELL™ POWERVAULT™ MD1XXX FAMILY SAS SOLUTION
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RAID Level Migration
Changing the RAID level of a virtual disk from the existing level (for example, RAID 1) to
another level (for example, RAID 5) is called RAID level migration. Dell recommends that
data in the virtual disk be backed up before a RAID level migration is initiated. Dell also
strongly recommends that the size of the physical disks to which the RAID level is to be
migrated is greater than or equal to the size of the physical disks in the existing virtual disk.
Dell supports RAID level migration as follows:
RAID 0 to RAID 1
RAID 0 to RAID 5
RAID 0 to RAID 6*
RAID 1 to RAID 0
RAID 1 to RAID 5
RAID 1 to RAID 6*
RAID 5 to RAID 0
RAID 5 to RAID 6*
RAID 6 to RAID 0*
RAID 6 to RAID 5
* RLM only supported on PERC 6/E only.
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Capacity Expansion
Expanding the capacity of a virtual disk by adding another physical disk is called
capacity expansion. Dell supports capacity expansion on all the supported RAID levels
except RAID 1. Dell recommends that the data in the virtual disk be backed up before
capacity expansion is initiated. For more details on capacity expansion, see the user's
guide for your specific controller.
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Controller Write Cache Policy Options
Write–Back Caching
In write-back caching, data transfer is completed when the controller cache receives all
data from host for the write transaction. Write-back caching is faster than write through
caching. The main advantage of write-back caching is improved performance since the
controller does not have to wait for an acknowledgement from the physical disk before
proceeding. The battery backup unit for the controller is required to enable write-back
caching. For more details on write-back caching, see the user's guide for your specific
controller.
Write-Through Caching
In write-through caching, a data transfer is completed when the disk subsystem receives
all of the data from the host. Write-through caching provides additional security because
data must be committed to the physical disk before proceeding. There may be a minimal
performance impact since the controller must wait for the physical disk to return a good
status to the controller before proceeding to the next operation.
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Hot Spare Support
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VER A02
6/16/2008