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[linux-lvm] Re: IO scheduler, queue depth, nr_requests



On Thu, 19 Feb 2004 03:26:28, Andrew Morton wrote:
> Miquel van Smoorenburg <miquels cistron nl> wrote:
> >
> > The thing is, the bio's are submitted perfectly sequentially. It's just that
> >  every so often a request (with just its initial bio) gets stuck for a while.
> >  Because the bio's after it are merged and sent to the device, it's not
> >  possible to merge that stuck request later on when it gets unstuck, because
> >  the other bio's have already left the building so to speak.
> 
> Oh.  So the raid controller's queue depth is larger than the kernel's.  So
> everything gets immediately shovelled into the device and the kernel is
> left with nothing to merge the little request against.

Well, the request queue of the kernel is max'ed out too, otherwise
get_request_wait() wouldn't be called. It's just an unfortunate timing issue.

> Shouldn't the controller itself be performing the insertion?

Well, you would indeed expect the 3ware hardware to be smarter than that,
but in its defence, the driver doesn't set sdev->simple_tags or sdev->ordered_tags
at all. It just has a large queue on the host, in hardware.

Perhaps this info should be exported into the request queue of the device,
so that ll_rw_blk knows about this and can do something similar to the
hack I posted ?

Note that AFAICS nothing in drivers/scsi uses the tagging stuff in ll_rw_blk.c.
blk_queue_init_tags() is only called by scsi_activate_tcq(), and nothing
ever calls that (except the 53c700.c driver). So you can't just check for
QUEUE_FLAG_QUEUED. Hmm, nothing in drivers/block calls it either. It's
not being used at all yet ? Or am I being dense ?

Mike.



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