Messages in this thread | | | Date | Tue, 22 Mar 2011 20:54:06 +0100 | From | Jens Axboe <> | Subject | Re: merging discard request in the block layer |
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On 2011-03-22 20:47, Christoph Hellwig wrote: > It seems the current block layer wil happily try to merge discard > requests that were split because they are at the max that bi_size > can hold together again. At least that's what the > > blk: request botched
That would seem to indicate a bug in the merging logic instead.
> make me believe when testing XFS code that allows multiple > asynchronous discard request, unlike the current blkdev_issue_discard > which always waits for one before starting the next. > > I tried this little sniplet to prevent it: > > Index: xfs/block/blk-merge.c > =================================================================== > --- xfs.orig/block/blk-merge.c 2011-03-22 13:07:24.733857580 +0100 > +++ xfs/block/blk-merge.c 2011-03-22 13:08:17.448856577 +0100 > @@ -373,7 +373,7 @@ static int attempt_merge(struct request_ > /* > * Don't merge file system requests and discard requests > */ > - if ((req->cmd_flags & REQ_DISCARD) != (next->cmd_flags & REQ_DISCARD)) > + if ((req->cmd_flags & REQ_DISCARD) || (next->cmd_flags & REQ_DISCARD)) > return 0; > > /*
That's not going to be enough, you want to disable the bio to request merging of discards as well in elevator.c:elv_rq_merge_ok(). Does that then fix it?
-- Jens Axboe
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