Exadata Flash Cache WriteBack
Posted by Vishal Gupta on Oct 2, 2012
With the announcement Exadata X3, Oracle has introduced a new feature called “FlashCache Writeback” to allow writes to cell Flash Cache (aka Exadata Smart FlashCache) in WriteBack mode. Earlier with WriteThrough mode, writes were not written to FlashCache, instead they were written directly to cell disks. Exadata software used to decide whether to cache these writes back into FlashCache or not. In WriteBack mode, writes are written to cell FlashCache and acknowledgement is given back to calling process as soon as data is written to flashcache. Exadata Server software de-stages the dirty writes in flashcache to spinning disks in the background. Writes to flashcache are persistent, so there will be no data loss in case of cell node failures.
Advantages of FlashCache WriteBack
- It can significantly improve database write speeds, as write to PCI FlashCards is faster than spinning disks by a order of magnitude.
- It will improve the speed of database checkpoints.
- Control files writes will be automatically done to flashcache, which can improve performance of logswitches, checkpoints.
- It can significantly improve direct path writes i.e. when sort/merge/hash join operation spill over from PGA memory to temporary tablespaces, they can be written to FlashCache instead of being written to slower spinning disks. This will speed large parallel query operations.
- Now there is no need to create separate ASM DiskGroup with disks carved out of flash disks to hosts tablespace (temporary or permanent) which require faster read/write response times. FlashCache WriteBack feature eliminates this need.
- Exadata V2/X2 full rack has 5.3TB of FlashCache and X3 has 22.4 TB of flashcache, which can be used for caching writes. This same FlashCache will be shared for both reads and writes, pretty much like what most of the SAN arrays do to use same RAM cache for “write pending cache” and “read cache”.
- Online read logs were already being written to FlashCache via FlashLogs, where by every write to online redo logs is done to both flashcache and disks. As soon as write completes on either of media, control is returned back to LGWR immediately. In some outlier cases, writes to flashcache can be slower in which case if write to disk finishes first, control is returned back to LGWR. This reduced waits on “logfile parallel write” event. FlashLog feature has been available since Exadata Server Software version 126.96.36.199.0 and above.
Good news is FlashCache WriteBack feature has been back ported to Exadata V2, X2, X2-8 models as well via Exadata Server Software version 188.8.131.52.0. You can improve the performance of your existing Exadata V2, X2-2 and X2-8 models by 10 times for FREE by simply upgrading the software. No need to swap out older Exadata models with new ones to get the performance boost. This is the part I like most.
Bad news is FlashCache WriteBack feature will only work if Grid Infrastructure and Database software version are at 184.108.40.206 BP9 (220.127.116.11.9) and above. This is because 18.104.22.168.9 or later include fixes for following bugs which are the marker bugs to add Flash Cache Write-Back support to ASM and RDBMS software.
Bug 14132953 Enhanacement to add Write-back flash cache resilvering support
Bug 14143451 Enhancement for ASM write-back flash cache resilvering support
Oh well, before older versions of GI and database software go out of support, one needs to upgrade them to latest patchset in any case, so bad news is not so bad after all.