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Re: Proposal: B_ARRIER (addresses wapbl performance?)

On Tue, Dec 09, 2008 at 09:16:32PM +0100, Manuel Bouyer wrote:
> On Tue, Dec 09, 2008 at 02:47:50PM -0500, Thor Lancelot Simon wrote:
> > [...]
> > Note that the default tag reordering scheme isn't supposed to reorder
> > even simple-tagged commands,
> Are you sure about this ? The way I read SAM-3, all simple task queued
> between 2 ordered task or head of queue task can complete in any order.
> Even if the write cache is disabled.

This depends on the value of QUEUE ALGORITHM MODIFIER in the Control
mode page.

See table 247 in SPC3:
and the descriptive text which immediately follows it (page 285 of the
PDF file).

If we really think we are using ORDERED tags everywhere we should, then
we should be setting this bit to 1 in the mode page, which should give
performance that in no way differs from running with WCE.  Elsewise, the
SIMPLE tags are reordered for read, but treated as ORDERED for write.

But this still exposes enough data to the drive that it should be able
to efficiently clear writes, if they are effectively sorted by the host.
If they are not, however, with QUEUE ALGORITHM MODIFIER set to 0 the
overall performance should be considerably worse than with WCE set.

Perhaps that is what Jason has observed.

It is unfortunate that the initiator can't create new task sets at
will.  If the scope of ORDERED tags were only over initiator-defined
task sets, we'd have exactly what we wanted.


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