Dell PowerVault MD3820f - Manual (Page 84)

4. In the Cache Properties area, you can select:
– Enable read caching
– Enable write caching
* Enable write caching without batteries — to permit write caching to continue even if the
RAID controller module batteries are discharged completely, not fully charged, or are not
present.
* Enable write caching with mirroring — to mirror cached data across two redundant RAID
controller modules that have the same cache size.
– Enable dynamic cache read prefetch
CAUTION: Possible loss of data—Selecting the Enable write caching without batteries option
lets write caching continue even when the batteries are discharged completely or are not
fully charged. Typically, write caching is turned off temporarily by the RAID controller
module until the batteries are charged. If you select this option and do not have a universal
power supply for protection, you could lose data. In addition, you could lose data if you do
not have RAID controller module batteries and you select the Enable write caching without
batteries option.
NOTE: When the Optional RAID controller module batteries option is enabled, the Enable write
caching does not appear. The Enable write caching without batteries is still available, but it is
not checked by default.
NOTE: Cache is automatically flushed after the Enable write caching check box is disabled.
5. Click OK.
A message prompts you to confirm the change in the virtual disk modification priority.
6. Click Yes.
7. Click OK.
The Change Virtual Disk Propreties - Progress dialog is displayed.
Changing The Segment Size Of A Virtual Disk
You can change the segment size on a selected virtual disk. During this operation, I/O performance is
affected, but your data remains available.
Follow these guidelines to proceed with changing the segment size:
• You cannot cancel this operation after it starts.
• Do not start this operation unless the disk group is in Optimal status.
• The MD Storage Manager determines the segment size transitions that are allowed. Segment sizes
that are inappropriate transitions from the current segment size are unavailable on the menu. Allowed
transitions usually are double or half of current segment size. For example, if the current virtual disk
segment size is 32 KB, a new virtual disk segment size of either 16 KB or 64 KB is allowed.
NOTE: The operation to change the segment size is slower than other modification operations (for
example, changing RAID levels or adding free capacity to a disk group). This slowness is the result of
how the data is reorganized and the temporary internal backup procedures that occur during the
operation.
The amount of time that a change segment size operation takes depends on:
• The I/O load from the host
• The modification priority of the virtual disk
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