On Jul 15, 2010, at 9:30 AM, Scott Carey wrote: >> Many raid controllers are smart enough to always turn off write caching on >> the drives, and also disable the feature on their own buffer without a BBU. >> Add a BBU, and the cache on the controller starts getting used, but *not* >> the cache on the drives. > > This does not make sense. > Write caching on all hard drives in the last decade are safe because they > support a write cache flush command properly. If the card is "smart" it > would issue the drive's write cache flush command to fulfill an fsync() or > barrier request with no BBU.
You're missing the point. If the power dies suddenly, there's no time to flush any cache anywhere. That's the entire point of the BBU - it keeps the RAM powered up on the raid card. It doesn't keep the disks spinning long enough to flush caches. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
