[linux-lvm] Re: IO scheduler, queue depth, nr_requests
Miquel van Smoorenburg
miquels at cistron.nl
Thu Feb 19 10:23:01 UTC 2004
On Thu, 19 Feb 2004 03:26:28, Andrew Morton wrote:
> Miquel van Smoorenburg <miquels at 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 ?
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