The Flash cache KEEP option

Oracle’s database flash cache (AKA “database smart flash cache”) supports selective caching at the table level via the FLASH_CACHE setting in the storage clause.

Settings can be DEFAULT, KEEP or NONE.  Eg:

The word KEEP is a bit unfortunate, since it perhaps implies behaviour identical to the KEEP pool of the buffer cache.  The KEEP cache is a separately managed area of specific size, so that you can control what proportion of your memory is allocated to “kept” blocks.   The flash cache KEEP setting is more to prioritise caching – there seems to be no limit on how many blocks will be kept.

For the above three tables, if we read all the rows in each more or less simultaneously, we get the following outcomes (shown here in our lovely new Spotlight on Oracle flash cache drilldown):

Not surprisingly,  where FLASH_CACHE is NONE, there is no flash cache storage.  When flash cache is KEEP, all the blocks are kept in the flash cache, and consequently there is not enough room for all the blocks from tables where FLASH_CACHE=DEFAULT.  However, unlike the KEEP buffer pool, there’s no way to limit the amount of blocks in the KEEP pool, and so you can quite easily fill up the cache with blocks form FLASH_CACHE=KEEP tables.  Once that happens, those blocks will stay there forever.

If you have Spotlight (or at least the 7.5 version about to be released with Flash cache diagnostics), you can see this happening.  Once the flash cache fills up, every new block introduced forces an old block out.  This shows up as evictions in the following chart:

Oracle clearly evicts blocks from FLASH_CACHE=DEFAULT tables in preference to FLASH_CACHE=KEEP.  Presumably (I haven't tested this) within each setting blocks are moved out using some form of Least Recently Used (LRU) or Not Recently Used algorithm. 

KEEP should therefore be used only for relatively small tables.  If you set FLASH_CACHE=KEEP for a large table (larger than the flash cache for instance) then you risk rendering the flash cache unusable for other tables.   I hope Oracle chooses to implement a limit on KEEP size in a subsequent release.  I think it would be far more useful if you could allocate multiple flash cache pools of a certain size as for KEEP and RECYCLE in the buffer cache.

Spotlight 7.5 – which includes flash cache diagnostics as well as ESX diagnostics, support for the 11gR2 parallel query queue and lots of other cool stuff - will be available next month, both on it’s own and as part of the TOAD DBA suite.