Dell PowerEdge RAID Controller 5i - Manual (Page 61)

RAID Configuration and Management
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When you define the virtual disks, you can set the virtual disk parameters described in Table 5-6.
• Stripe element size
• Write policy
• Read policy
Table 5-6. Virtual Disk Parameters and Descriptions
Parameter Description
Stripe Element
Size Stripe Element Size specifies the size of the segments written to each physical disk in a RAID 0,
1, 5, 10, and 50 virtual disk. You can set the stripe element size to 8 KB, 16 KB, 32 KB, 64 KB, or
128 KB. The default and recommended stripe element size is 128 KB.
A larger stripe element size provides better read performance, especially if your system does
mostly sequential reads. However, if you are sure that your system does random read requests with
small file sizes, select a small stripe element size.
Write Policy Write Policy specifies the controller write policy. You can set the write policy to Write-Back or
Write-Through.
In Write-Back caching, the controller sends a data transfer completion signal to the host when the
controller cache has received all the data in a transaction.
NOTE:
The default cache setting is Write-back cache.
NOTICE:
If Write-back is enabled and the system is quickly turned off and then on, the controller may
pause as the system flushes cache memory. Controllers that contain a battery backup will default to
write-back caching.
In Write-Through caching, the controller sends a data transfer completion signal to the host when
the disk subsystem has received all the data in a transaction.
NOTE:
Certain data patterns and configurations perform better with Write-through cache policy.
Write-Back caching has a performance advantage over write-through caching.
Read Policy Read-ahead enables the read-ahead feature for the virtual disk. You can set this parameter to
Read-ahead, No-read-ahead, or Adaptive. The default is No-read-ahead.
Read-ahead specifies that the controller uses read-ahead for the current virtual disk. Read-ahead
capability allows the controller to read sequentially ahead of requested data and store the
additional data in cache memory, anticipating that the data will be needed soon. Read-ahead
supplies sequential data faster, but is not as effective when accessing random data.
No-read-ahead specifies that the controller does not use read-ahead for the current virtual disk.
NOTE:
No-read-ahead shows higher performance results due to the effectiveness of hard-drive
caching algorithms.
Adaptive specifies that the controller begins using read-ahead if the two most recent disk
accesses occurred in sequential sectors. If all read requests are random, the algorithm reverts to
No-read-ahead; however, all requests are still evaluated for possible sequential operation.
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