Re: Maximum transaction rate

Joshua D. Drake wrote:
> On Sat, 2009-03-14 at 05:25 +0100, Marco Colombo wrote:
>> Scott Marlowe wrote:
>
>> Also see:
>> http://lkml.org/lkml/2008/2/26/41
>> but it seems to me that all this discussion is under the assuption that
>> disks have write-back caches.
>> "The alternative is to disable the disk write cache." says it all.
>
> If this applies to raid based cache as well then performance is going to
> completely tank. For users of Linux + PostgreSQL using LVM.
>
> Joshua D. Drake

Yet that's not the point. The point is safety. I may have a lightly loaded
database, with low write rate, but still I want it to be reliable. I just
want to know if disabling the caches makes it reliable or not. People on LK
seem to think it does. And it seems to me they may have a point.
fsync() is a flush operation on the block device, not a write barrier. LVM
doesn't pass write barriers down, but that doesn't mean it doesn't perform
a flush when requested to.

.TM.