> Any good SCSI drive knows the physical geometry of the disk and can
> therefore optimally schedule reads and writes.
If the scheduler inside the OS can rely on the drive to have a linear
mapping it is implemented as a cyclical-scan scheduler, there is only
a little difference in the effectivity of the scheduling.
> Although necessary features, like read queueing, are also available
> in the current SATA spec, I'm not sure most drives will implement it,
> at least not very well.
AFAIK the Seagate Barracudas implement first-party-dma, which is a
very lightweight implementation of command-queuing; with first-party
-dma, the CPU includes the adresses of where the data for a certain
request is read or written into the request-command so that when the
disk responds to a command, it sends this information back to the
controller. So this chip doesn't have to hold a list of outstanding
requests and their scattered adresses where the data is read from or
written to. Doesn't look very elegant, but that's just in line with
the cheap ATA-concepts.
And the only controller-chips I'm aware of supporting this features
are the chips fom Silicon Image. But I think that sooner or later,
all SATA-chips will support it.
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This archive was generated by hypermail 2b29 : Tue Apr 15 2003 - 22:00:27 EST