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    DateThu, 27 Dec 2007 21:23:58 +0800
    From"jidong xiao" <>
    SubjectRe: What does "ordering by draining" mean?
    On Dec 10, 2007 6:37 PM, Jens Axboe <jens.axboe@oracle.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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