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Re: [Cluster-devel] [PATCH 2/4] gfs2: ordered writes are backwards


This looks good. There is an argument for trying to sort the buffers as
we write them (in case the application writes them out of order) but
this seems a sensible change to catch 90% of cases. I'm just about to
give this a quick test and I'll push this one in straight away if it
looks good on my test,


On Fri, 2010-02-05 at 16:45 +1100, Dave Chinner wrote:
> When we queue data buffers for ordered write, the buffers are added
> to the head of the ordered write list. When the log needs to push
> these buffers to disk, it also walks the list from the head. The
> result is that the the ordered buffers are submitted to disk in
> reverse order.
> For large writes, this means that whenever the log flushes large
> streams of reverse sequential order buffers are pushed down into the
> block layers. The elevators don't handle this particularly well, so
> IO rates tend to be significantly lower than if the IO was issued in
> ascending block order.
> Queue new ordered buffers to the tail of the ordered buffer list to
> ensure that IO is dispatched in the order it was submitted. This
> should significantly improve large sequential write speeds. On a
> disk capable of 85MB/s, speeds increase from 50MB/s to 65MB/s for
> noop and from 38MB/s to 50MB/s for cfq.
> Signed-off-by: Dave Chinner <dchinner redhat com>
> ---
>  fs/gfs2/lops.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> diff --git a/fs/gfs2/lops.c b/fs/gfs2/lops.c
> index 5708edf..7278cf0 100644
> --- a/fs/gfs2/lops.c
> +++ b/fs/gfs2/lops.c
> @@ -532,9 +532,9 @@ static void databuf_lo_add(struct gfs2_sbd *sdp, struct gfs2_log_element *le)
>  		gfs2_pin(sdp, bd->bd_bh);
>  		tr->tr_num_databuf_new++;
>  		sdp->sd_log_num_databuf++;
> -		list_add(&le->le_list, &sdp->sd_log_le_databuf);
> +		list_add_tail(&le->le_list, &sdp->sd_log_le_databuf);
>  	} else {
> -		list_add(&le->le_list, &sdp->sd_log_le_ordered);
> +		list_add_tail(&le->le_list, &sdp->sd_log_le_ordered);
>  	}
>  out:
>  	gfs2_log_unlock(sdp);

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