Dell PowerVault MD3000i Dell PowerVault MD3000/MD3000i Array Tuning Best Pract - 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 WriteThrough 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: 13+1 RAID 5 User Label: MyRAID5_1 Block Size: 512 bytes Large IO: 4096 blocks Segment Size: 256 blocks Stripe Size: 3328 blocks ... IO Statistics: small small reads writes requests 2028332119 147699066 blocks 3091968111 2518067526 avg blocks 4 17 IO pct. 93.21% 6.78% large reads 0 0 0 0.00% large cache writes total hits 0 2176031185 1289775370 0 1315068341 4019884678 0 0 3 0.00% 0.00% 59.27% IOs stripes reads 2028332119 2034477363 writes 147699066 148449472 /IO clusters 1.00 2107869128 1.00 157404718 /IO 1.03 1.06 write algorithms Full Partial RMW No Parity 1105611 12598366 32120072 0 RMW2 0 FSWT 0 December 2008 - Revision A01 Page 20