Date: Fri, 27 Oct 2006 15:56:33 -0600 From: Scott Long <scottl@samsco.org> To: Martin Cracauer <cracauer@cons.org> Cc: freebsd-scsi@freebsd.org Subject: Re: Why is SCSI so much faster with the write cache off (than ATA)? Message-ID: <45428091.9030208@samsco.org> In-Reply-To: <20061027214011.GB86642@cons.org> References: <20061027214011.GB86642@cons.org>
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Martin Cracauer wrote: > I have observed it several times and I am missing one bit of knowledge > here: > > why is SCSI so much faster when you turn off the write cache than > P-ATA and SATA? > > P-ATA and SATA crumble to about 1/10th of the speed (just writing one > file with 8k blocks linear), whereas SCSI just loses 10-20%, for me. > I have observed that 10 years ago with some 8 GB ATA IBM disk on the > BX chipset versus some 4 GB Quantum Atlas, and now I see it again with > Seagate 7200.7s and .8s versus a 10K Compaq labled 36 GB drive. > > Personally I don't see why a linear write should be slow at all. > Surely the computer delivers the data fast enough for sectors to be > filled as they pass under the head. Maybe the ATA disks lose one > rotation per sector or per filesystem block written anyway? Then why > don't SCSI disks lose the same way given they are not allowed to cache > anything either? > > Martin The answer: Tagged Queueing The drive can collect a bunch of I/O requests, sort them for optimal layout, then stream their data in all at once. This is not the same thing as buffering through the write cache, but it has a similar performance benefit. Scott
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