Re: Weird XFS WAL problem

On Thu, Jun 3, 2010 at 1:31 PM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
> Scott Marlowe wrote:
>>
>> I think it's a case of the quickest, simplest answer to semi-new tech.
>>  Not sure what to do with barriers?  Just flush the whole cache.
>>
>
> Well, that really is the only useful thing you can do with regular SATA
> drives; the ATA command set isn't any finer grained than that in a way
> that's useful for this context.  And it's also quite reasonable for a RAID
> controller to respond to that "flush the whole cache" call by flushing its
> cache.  So it's not just the simplest first answer, I believe it's the only
> answer until a better ATA command set becomes available.
>
> I think this can only be resolved usefully for all of us at the RAID
> firmware level.  If the controller had some logic that said "it's OK to not
> flush the cache when that call comes in if my battery is working fine",

That's what already happens for fsync on a BBU controller, so I don't
think the code to do so would be something fancy and new, just a
simple change of logic on which code path to take.