256MB cache module, no battery

[Allen Huang_1]

We have systems with 256MB cache module but no battery. Without the battery to backup these cache’s, should we leave off write caching?
If that is the case, is the memory module actually doing anything?

Is it true that the battery protects the cache during a power failure and the memory module is used as the cache for the controller?

[miaso120]

By default if you have cache module, but no battery backup module, all the memory is allocated for read cache. To enable also write caching you have to either install a battery backup module or to go to controller settings and mark a checkbox to force write cache even without battery (this may be availible not on all controllers)

Enabling write caching may greatly improve the performance of disk subsystem depending on type of your workload. But there might be a situation when OS writes some data to disk, it gets to cache but not yet to disk. If at this point server gets shutdown (power loss, BSOD, etc..) the data waiting to be written to disk in cache is lost - this incomplete write operation could possibly corrupt your data. Battery backup is used to prevent this situation. A small separate battery can provide power to cache module for up to 72 hours (typical time). If during this 3 days you power up the server or transfer the controller with disk to another server - the cache flushes to disks and your data is saved. So its highly discouraged to use write cache without battery backup module (and many controllers even dont allow you to do so)

[Ali · Accepted solution]

HI Allen,

what is the server and controller model?

Smart array controller uses the following Caching :
Read-ahead caching
Write-back caching

the term that we use for smart array controllers is BBWC (battery backed write cache) which includes Cache, Battery and Battery cable.

BBWC provides write-back cache functionality that accelerates writing by storing chunk of data and writing it back to harddrives this way it achieves better write IO operations. Data stored in Cache during IO operations are backed by battery incase of power failure. it can retain data up to 72 hours.

Just A cache memory works on Write through mechanism where data is simultaneously written back to hard drives. When there is no Battery attached to the Cache connected to smart array controller it work on read-ahead caching. This adaptive read-ahead scheme provides excellent performance for sequential small block read requests.

BBWC can be configure to take advantage of write-back and read-ahead caching which acelerates read and write both performance.

Just a cache give better read performance.
for more details, please refer to page no 8 of HP Smart array controller technology guide:
http://h20000.www2.hp.com/bc/docs/support/SupportManual/c00687518/c00687518.pdf

hope this helps,

thanks,
Aftab
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