With the 18.104.22.168.0 release of the Exadata storage server software (and providing you are at least at 22.214.171.124 BP11), you will have the opportunity to utilise Exadata Smart Flash Logging. I thought I’d take a look at how much (if any) improvement this feature would provide to a busy production environment.
Have a look at this blog entry on Exadata Smart Flash Loggin by Luis Moreno Campos for an introduction on how it works. Basically you now issue two writes, 1 to flash 1 to your disk based redo logs, fastest write is the winner.
First lets check that we actually have some Exadata Smart Flash Log’s available to be used:
CellCLI> list flashlog detail name cel07_FLASHLOG cellDisk FD_00_cel07,FD_08_cel07,FD_09_cel07,FD_01_cel07,FD_15_cel07,FD_06_cel07,FD_03_cel07,FD_04_cel07,FD_07_cel07,FD_02_cel07,FD_12_cel07,FD_14_cel07,FD_13_cel07,FD_11_cel07,FD_05_cel07,FD_10_cel07 creationTime 2012-03-17T15 degradedCelldisks effectiveSize 512M efficiency 100.0 id a24a25e5-062e-4be1-bb6b-3168113a5fe8 size 512M status normal
First we can see that on this cell, there is a flashlog created of size 512M. It is carved out of each the 16 flash doms in the cell. Consequently this reduces the amount you have for your flashcache, though it’s a very small reduction.
How much use are we getting out of the flashlog? Well, we can look at some metrics:
CellCLI> list metriccurrent where objectType='FLASHLOG' FL_ACTUAL_OUTLIERS FLASHLOG 0 IO requests FL_BY_KEEP FLASHLOG 0 FL_DISK_FIRST FLASHLOG 6,168,815 IO requests FL_DISK_IO_ERRS FLASHLOG 0 IO requests FL_EFFICIENCY_PERCENTAGE FLASHLOG 100 % FL_EFFICIENCY_PERCENTAGE_HOUR FLASHLOG 100 % FL_FLASH_FIRST FLASHLOG 172,344 IO requests FL_FLASH_IO_ERRS FLASHLOG 0 IO requests FL_FLASH_ONLY_OUTLIERS FLASHLOG 0 IO requests FL_IO_DB_BY_W FLASHLOG 286,075 MB FL_IO_DB_BY_W_SEC FLASHLOG 13.328 MB/sec FL_IO_FL_BY_W FLASHLOG 303,793 MB FL_IO_FL_BY_W_SEC FLASHLOG 13.761 MB/sec FL_IO_W FLASHLOG 6,341,159 IO requests FL_IO_W_SKIP_BUSY FLASHLOG 0 IO requests FL_IO_W_SKIP_BUSY_MIN FLASHLOG 0.0 IO/sec FL_IO_W_SKIP_LARGE FLASHLOG 0 IO requests FL_PREVENTED_OUTLIERS FLASHLOG 415 IO requests
First off, this is taken on a very busy system:
FL_IO_FL_BY_W_SEC: 13.761 MB/sec
That is saying how much data is being written to flash by smart flash log. Well, that sounds great, but it’s not quite so simple. Remember writes go to both flash and disk.
FL_DISK_FIRST: 6,168,815 IO requests
This metric is actually telling that 6.1M I/O requests were serviced first by disk. While:
FL_FLASH_FIRST: 172,344 IO requests
Is saying this number went to flash first. Oh, that’s not a great improvement! I make that 2.7% writes went to flash first.
Finally a word on the FL_PREVENTED_OUTLIERS, this is saying there was 415 writes on this cell that would have taken more than 0.5 secs if there was no flash logging in place.
I have also checked AWR reports on before and after having flash logging in place. There is very little change. AWR shows an average wait of 3ms for Log File Parallel Write. Have a look at the wait event histogram for this:
We see the vast majority of writes are under a 1ms. This was also the case before the flash logging as well. It has not improved this at all.
This is a busy cpu bound system lets look at the Log file sync wait event histogram:
Eurgh! is the only way to describe this.
I think Kevin Closson has covered this a mere half-decade ago!