Does it really matter at which end of the cable the queueing is done
(Assuming both ends know as much about drive geometry etc..)?
Alex Turner
netEconomist
On 4/18/05, Bruce Momjian <pgman@candle.pha.pa.us> wrote:
> Kevin Brown wrote:
> > Greg Stark wrote:
> >
> >
> > > I think you're being misled by analyzing the write case.
> > >
> > > Consider the read case. When a user process requests a block and
> > > that read makes its way down to the driver level, the driver can't
> > > just put it aside and wait until it's convenient. It has to go ahead
> > > and issue the read right away.
> >
> > Well, strictly speaking it doesn't *have* to. It could delay for a
> > couple of milliseconds to see if other requests come in, and then
> > issue the read if none do. If there are already other requests being
> > fulfilled, then it'll schedule the request in question just like the
> > rest.
>
> The idea with SCSI or any command queuing is that you don't have to wait
> for another request to come in --- you can send the request as it
> arrives, then if another shows up, you send that too, and the drive
> optimizes the grouping at a later time, knowing what the drive is doing,
> rather queueing in the kernel.
>
> --
> Bruce Momjian | http://candle.pha.pa.us
> pgman@candle.pha.pa.us | (610) 359-1001
> + If your life is a hard drive, | 13 Roberts Road
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