Date: Tue, 12 Oct 2004 11:33:15 -0400 From: David Gilbert <dgilbert@dclg.ca> To: "Kenneth D. Merry" <ken@freebsd.org> Cc: freebsd-hardware@freebsd.org Subject: Re: sata raid & write cache state Message-ID: <16747.63803.470649.921882@canoe.dclg.ca> In-Reply-To: <20041011210303.GA78436@nargothrond.kdm.org> References: <b21e6cca041010181932879aeb@mail.gmail.com> <20041011043508.GA72113@nargothrond.kdm.org> <b21e6cca041011090816a1352@mail.gmail.com> <20041011210303.GA78436@nargothrond.kdm.org>
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>>>>> "Kenneth" == Kenneth D Merry <ken@freebsd.org> writes: Kenneth> See my previous mail. SATA disks differ in two ways: Kenneth> 1. Many don't support tagged queueing. I'd like to see more information on this. I was under the impression that SATA required some form of command queueing in all drives. Kenneth> 2. If the SATA disk does support tagged queueing, there is Kenneth> still a fundamental problem with the queueing model in SATA Kenneth> (and probably ATA, not sure). According to a coworker of Kenneth> mine (hardware engineer) who is a SATA expert, the status Kenneth> phase on the bus is the same phase as the data phase. So you Kenneth> basically have to send all the data to the drive on a write Kenneth> and the drive has to send the status back before the drive Kenneth> can accept any more data for another queued write command. Kenneth> So that limits you, effectively, to writing data for one Kenneth> command at a time. It would appear that the SATA folks are introducing 'NCQ' (Native Command Queueing) ... which does supply out-of-order returns among other things. Dave. -- ============================================================================ |David Gilbert, Independent Contractor. | Two things can only be | |Mail: dave@daveg.ca | equal if and only if they | |http://daveg.ca | are precisely opposite. | =========================================================GLO================
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