Dell PowerVault MD3000i - Manual (Page 19)

Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices
December 2008 – Revision A01 Page 19
4.7.5 Write Algorithm Data
It is important to understand the effect determining the most suitable RAID level
can be a daunting task. Understanding the effect of which write algorithm is in
use is an important part of RAID Level balance. The possible choices in first
generation firmware, as seen in Figure 9, are Full, Partial, RMW, RMW2, and the
Full Stripe Write-Through. RMW2 was folded into RMW statistics in the second
generation firmware (see Figure 10).
The Full algorithm takes an entire stripe of data and dumps it to the disk,
depending on the RAID level of choice, P or P and Q will get calculated at this
point. This is the most efficient type of write that can be performed, and the
design of a disk group should be around maximizing full writes.
Partial writes are when less than a full stripe of data non-aligned with segment
boundaries are modified and written. In RAID level 5 and 6 this is more complex,
as parity data must be recalculated for the whole stripe. Partial writes are a
worst-case algorithm and should be minimized. Larger quantities of partial writes
than full writes can indicate use of an inappropriate segment size.
Volume 0 Attributes:
Volume Type: RAIDVolume
User Label: MyRAID10_One
...
BlockSize: 512 bytes
LargeIoSize: 4096 blocks
...
Perf. Stats: Requests Blocks Avg. Blks IO Percent
Reads 67456452 5943724625 88 71.20%
Writes 27283249 1144902648 41 28.80%
Large Reads 0 0 0 0.00%
Large Writes 0 0 0 0.00%
Total 94739701 7088627273 74 100.33%
...
*** Performance stats ***
Cluster Reads Cluster Writes Stripe Reads
6252626 3015009 5334257
Stripe Writes Cache Hits Cache Hit Blks
2040493 4685032 737770040
RPA Requests RPA Width RPA Depth
982036 3932113 418860162
Full Writes Partial Writes RMW Writes
653386 29 328612
No Parity Writes Fast Writes Full Stripe WT
0 0 0
1.
2.
2.
3.
Figure 8: Second Generation Firmware– Stripe distribution. File:
stateCaptureData.txt