sata raid & write cache state

[David Gilbert]

>>>>> "Kenneth" == Kenneth D Merry <ken at 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.

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|David Gilbert, Independent Contractor.       | Two things can only be     |
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