Dell PowerVault MD3000i Tuning Manual page 20

Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices
RMW, or Read-Modify-Write, is the second-best write algorithm available for
RAID 5 and 6. A RMW occurs when a quantity of bits, smaller or equal to an
individual segment are modified. This constitutes a two-read operation in RAID 5
and a three-read operation in RAID 6, with one of the segments being modified,
and the parity drive(s) are read in. Then parity for the affected region is
recalculated and then the data and parity in the stripe are re-written to disk. In
small transactional processing, an extremely large number of RMWs should be
expected. These RMW writes can cause a significant loss of performance;
however this impact can be lessened by proper tuning of virtual disk stripe size. .
RMW2 is used to differentiate Write-to-cache RMWs and Write-Through RMWs,
with RMW2 being the latter, these statistics were consolidated in the second
generation firmware. RMW2's also specifically happen when cache is disabled
by force, failed mirroring controller (if policy is active) or failed cache battery.
Additionally, the second generation firmware discretely tracks Full Stripe Write-
Through conditions, and both generations track data on Number of Parity stripes
re-calculated.
Figure 9: First Generation Firmware - Write Algorithms
Virtual Disk Unit 0 Configuration
Volume Type:
User Label:
Block Size:
Large IO:
Segment Size:
Stripe Size:
...
IO Statistics:
small
reads
requests 2028332119
blocks 3091968111 2518067526
avg blocks
IO pct. 93.21%
reads 2028332119 2034477363
writes 147699066
write Full
algorithms 1105611
December 2008 – Revision A01
13+1 RAID 5
MyRAID5_1
512 bytes
4096 blocks
256 blocks
3328 blocks
small
writes
147699066
4 17
6.78%
IOs stripes
148449472
Partial
12598366 32120072
large large
reads writes
0 0 2176031185 1289775370
0 0 1315068341 4019884678
0 0
0.00% 0.00%
/IO clusters
1.00 2107869128
1.00 157404718
RMW No Parity
0
cache
total hits
0 3
0.00% 59.27%
/IO
1.03
1.06
RMW2 FSWT
0 0
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