Re: SSD + RAID

Greg Smith wrote:
> Bruce Momjian wrote:
>> I always assumed SCSI disks had a write-through cache and therefore
>> didn't need a drive cache flush comment.

Some do. SCSI disks have write-back caches.

Some have both(!) - a write-back cache but the user can explicitly
send write-through requests.

Microsoft explains it well (IMHO) here:
http://msdn.microsoft.com/en-us/library/aa508863.aspx
"For example, suppose that the target is a SCSI device with
a write-back cache. If the device supports write-through
requests, the initiator can bypass the write cache by
setting the force unit access (FUA) bit in the command
descriptor block (CDB) of the write command."

> this perception, which I've recently come to believe isn't actually
> correct anymore. ... I'm staring to think this is what
> we've all been observing rather than a write-through cache

I think what we've been observing is that guys with SCSI drives
are more likely to either
(a) have battery-backed RAID controllers that insure writes succeed,
or
(b) have other decent RAID controllers that understand details
like that FUA bit to send write-through requests even if
a SCSI devices has a write-back cache.

In contrast, most guys with PATA drives are probably running
software RAID (if any) with a RAID stack (older LVM and MD)
known to lose the cache flushing commands.