From: Ron Mayer <rm_pg@cheapcomplexdevices.com>
To: Greg Smith <greg@2ndquadrant.com>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: Ron Mayer <rm_pg@cheapcomplexdevices.com>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: SSD + RAID
Date: Wed, 03 Mar 2010 07:16:40 -0800
Message-ID: <4B8E7D58.9030208@cheapcomplexdevices.com> (raw)
In-Reply-To: <4B8CAC89.1020100@2ndquadrant.com>
References: <201003020333.o223Xrf20941@momjian.us>
<4B8CAC89.1020100@2ndquadrant.com>
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.
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To: pgsql-performance@postgresql.org
Cc: rm_pg@cheapcomplexdevices.com, greg@2ndquadrant.com, bruce@momjian.us
Subject: Re: SSD + RAID
In-Reply-To: <4B8E7D58.9030208@cheapcomplexdevices.com>
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