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. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
