Message-ID: <4B8E7D58.9030208@cheapcomplexdevices.com>
Date: Wed, 03 Mar 2010 07:16:40 -0800
From: Ron Mayer <rm_pg@cheapcomplexdevices.com>
User-Agent: Thunderbird 2.0.0.23 (X11/20090817)
MIME-Version: 1.0
To: Greg Smith <greg@2ndquadrant.com>
CC: Bruce Momjian <bruce@momjian.us>,
	Ron Mayer <rm_pg@cheapcomplexdevices.com>,
	pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: SSD + RAID
References: <201003020333.o223Xrf20941@momjian.us>
	<4B8CAC89.1020100@2ndquadrant.com>
In-Reply-To: <4B8CAC89.1020100@2ndquadrant.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

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.