On Thu, Jul 15, 2010 at 12:35 PM, Ben Chobot <be...@silentmedia.com> wrote:
> 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 > So you are saying write caching is a dangerous proposition on a raid card with or without BBU?
