linux-kernel - Re: What does "ordering by draining" mean?

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Message-ID: <4104961b0712270523g30d04560qb1b6df0e6aadbf4c@mail.gmail.com>
Date:	Thu, 27 Dec 2007 21:23:58 +0800
From:	"jidong xiao" <jidong.xiao@...il.com>
To:	"Jens Axboe" <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: What does "ordering by draining" mean?

On Dec 10, 2007 6:37 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
>
> On Mon, Dec 10 2007, jidong xiao wrote:
> > Hi,
> >
> > I am reading the source code of block I/O layer, and now seeing there
> > saying "ordering by draining" or "ordering by tag" in
> > include/blkdev.h,and the comments in that file says: "Hardbarrier is
> > supported with one of the following methods", but what do these two
> > methods exactly mean?
>
> ordering by draining means that you empty the pending queue of requests
> and then flush the cache to prevent a later issue of a new request to
> reach the platter before the older ones. order by tag means that the hw
> has a way of being told not to reorder beyond the barrier, in which case
> you don't have to drain the queue.
>

Well, I saw these comments in sd_revalidate_disk(), does this mean
SCSI TCQ is not supported in Linux?

   1545         /*
   1546          * We now have all cache related info, determine how we deal
   1547          * with ordered requests.  Note that as the current SCSI
   1548          * dispatch function can alter request order, we cannot use
   1549          * QUEUE_ORDERED_TAG_* even when ordered tag is supported.
   1550          */
   1551         if (sdkp->WCE)
   1552                 ordered = sdkp->DPOFUA
   1553                         ? QUEUE_ORDERED_DRAIN_FUA :
QUEUE_ORDERED_DRAIN_FLUSH;
   1554         else
   1555                 ordered = QUEUE_ORDERED_DRAIN;
   1556
   1557         blk_queue_ordered(sdkp->disk->queue, ordered, sd_prepare_flush);

Regards
Jason
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