On Thu, Jul 15, 2010 at 12:35 PM, Ben Chobot <bench@xxxxxxxxxxxxxxx> wrote:
On Jul 15, 2010, at 9:30 AM, Scott Carey wrote: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.
>> 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.
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So you are saying write caching is a dangerous proposition on a raid card with or without BBU?
