Poor RAID performance demystified

Ivan Voras ivoras at freebsd.org
Thu Nov 25 11:10:35 UTC 2010

On 11/25/10 10:20, Yar Tikhiy wrote:

> If you
> still need greater write performance on tiny transactions, consider
> getting a battery backup unit (BBU) for your RAID adapter.  Quite
> remarkably, HP refer to them as "Write-back Cache Enablers" because
> installing one is the only way to get an HP RAID adapter do write-back
> caching.  A write-back cache with BBU will let the adapter delay and
> coalesce tiny writes without jeopardizing the DB integrity.  However,
> you'll need to trust your BBU as your DB integrity will be staked on
> it (the PG folks are somehow skeptical about BBUs).

HP also has (and so do probably others by now) capacitor-backed flash 
caches; the theory is to have a fast random IO chunk of flash memory and 
use the capacitor to keep the power up for as long as the flash needs to 
write its large blocks.

I've tried it and the performance is good, but don't have it in 
production yet.

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