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Date:      Sun, 6 May 2001 12:06:11 -0600
From:      "Kenneth D. Merry" <ken@kdm.org>
To:        Jan Conrad <conrad@th.physik.uni-bonn.de>
Cc:        freebsd-scsi@FreeBSD.ORG
Subject:   Re: Write Cacheing on SCSI controllers?
Message-ID:  <20010506120611.C94763@panzer.kdm.org>
In-Reply-To: <Pine.BSF.4.33.0105061509510.84550-100000@merlin.th.physik.uni-bonn.de>; from conrad@th.physik.uni-bonn.de on Sun, May 06, 2001 at 03:35:29PM %2B0200
References:  <Pine.BSF.4.33.0105061509510.84550-100000@merlin.th.physik.uni-bonn.de>

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On Sun, May 06, 2001 at 15:35:29 +0200, Jan Conrad wrote:
> Hi,
> 
> I am setting up a new file server right now so I have some time to play
> around setting WCE on or off.
> 
> To my surprise it did'nt matter that much! Just dd'ing 400MB of zeros to
> the disc gives me about 32MB/s, (nearly) no matter how WCE is set.
> (29160, IBM DDYS-T18350N) (softupdates on/off does'nt matter that much
> either...)
> 
> This is in sharp contrast to ATA disks, where the write throughput drops
> by a factor of 3 when disabling WC.
> 
> Is it possible that the SCSI controller does the write cacheing,
> compensating for the drive not doing it?

I don't know of any modern SCSI controllers that do caching.  The 29160
definitely doesn't cache.  (RAID controllers are another story.) So no,
the controller isn't doing it.

As Joerg pointed out, what you're probably seeing is the effect of tagged
queueing.  With SCSI, (and with some ATA drives), tagged queueing gives you
the ability to queue a whole bunch of commands to the disk and then wait
for completion.

Without tagged queueing (often the case with ATA), you can only issue one
command to the disk and then you have to wait for completion.  If you've
got write caching turned on, though, the disk can acknowledge the command
immediately, even though it hasn't been committed to disk.  So the time
between writes goes down and you're better able to saturate the disk.

If you want to see a difference in performance with a SCSI disk with and
without write caching, try a whole bunch of random I/O.  (newfs used to be
a good test, although that may have been fixed recently.)

Ken
-- 
Kenneth Merry
ken@kdm.org

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