Re: performance on new linux box

On Thu, Jul 15, 2010 at 12:35 PM, Ben Chobot <bench(at)silentmedia(dot)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.
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So you are saying write caching is a dangerous proposition on a raid card
with or without BBU?