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SubjectRe: Scatter-gather list constraints
On Sun, 22 Jun 2008, Andi Kleen wrote:

> >> - Is it performance critical?
> >
> > For people using wireless USB drives, yes.
> But only if there is a lot of 512 byte block IO? The only case I can think
> of right now would be XFS log IO and perhaps some O_DIRECT/raw device
> accesses.

Sorry, I misunderstood your question. There probably will not be a lot
of 512-byte block I/O -- not in the workloads I'm acquainted with. But
there will be some.

It isn't performance-critical, in the sense that slowing down the odd
512-byte block transfers won't hurt performance much. But it is
critical in the sense that the transfers must work properly when they
do occur.

> If it's only an relative oddball just copying is fine imho.

You mean, have the USB stack allocate bounce buffers and copy the data
between the S-G buffers (which may be in high memory) and the bounce
buffers? We're talking about a potentially fairly large amount of
data, say up to 100 KB. Is that really easier than splitting up an I/O

Alan Stern

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