Re: [PATCH] rbd: fix I/O error propagation for reads

[Mike Dawson <mike.dawson@cloudapt.com>]

Josh,

The original bug is marked as krbd, but could this bug could affect rbd 
volumes mounted via qemu as well? If so, could you describe how it might 
block a qemu guest?

We've been fighting i/o issues on some of our guests for some time. With 
qemu 1.4.0, we saw the entire guest freeze. But now with qemu 1.5.2 
which includes your asynchronous flush patch, the issue is typified by 
periodic dips in performance and high latency (especially for reads, it 
seems). Could this bug be related?

Thanks,
Mike Dawson


On 8/26/2013 9:34 PM, Josh Durgin wrote:
> When a request returns an error, the driver needs to report the entire
> extent of the request as completed.  Writes already did this, since
> they always set xferred = length, but reads were skipping that step if
> an error other than -ENOENT occurred.  Instead, rbd would end up
> passing 0 xferred to blk_end_request(), which would always report
> needing more data.  This resulted in an assert failing when more data
> was required by the block layer, but all the object requests were
> done:
>
> [ 1868.719077] rbd: obj_request read result -108 xferred 0
> [ 1868.719077]
> [ 1868.719518] end_request: I/O error, dev rbd1, sector 0
> [ 1868.719739]
> [ 1868.719739] Assertion failure in rbd_img_obj_callback() at line 1736:
> [ 1868.719739]
> [ 1868.719739]   rbd_assert(more ^ (which == img_request->obj_request_count));
>
> Without this assert, reads that hit errors would hang forever, since
> the block layer considered them incomplete.
>
> Fixes: http://tracker.ceph.com/issues/5647
> Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
> ---
>   drivers/block/rbd.c |   14 +++++++-------
>   1 files changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
> index 0d669ae..f8fd7d3 100644
> --- a/drivers/block/rbd.c
> +++ b/drivers/block/rbd.c
> @@ -1557,11 +1557,12 @@ rbd_img_obj_request_read_callback(struct rbd_obj_request *obj_request)
>   		obj_request, obj_request->img_request, obj_request->result,
>   		xferred, length);
>   	/*
> -	 * ENOENT means a hole in the image.  We zero-fill the
> -	 * entire length of the request.  A short read also implies
> -	 * zero-fill to the end of the request.  Either way we
> -	 * update the xferred count to indicate the whole request
> -	 * was satisfied.
> +	 * ENOENT means a hole in the image.  We zero-fill the entire
> +	 * length of the request.  A short read also implies zero-fill
> +	 * to the end of the request.  An error requires the whole
> +	 * length of the request to be reported finished with an error
> +	 * to the block layer.  In each case we update the xferred
> +	 * count to indicate the whole request was satisfied.
>   	 */
>   	rbd_assert(obj_request->type != OBJ_REQUEST_NODATA);
>   	if (obj_request->result == -ENOENT) {
> @@ -1570,14 +1571,13 @@ rbd_img_obj_request_read_callback(struct rbd_obj_request *obj_request)
>   		else
>   			zero_pages(obj_request->pages, 0, length);
>   		obj_request->result = 0;
> -		obj_request->xferred = length;
>   	} else if (xferred < length && !obj_request->result) {
>   		if (obj_request->type == OBJ_REQUEST_BIO)
>   			zero_bio_chain(obj_request->bio_list, xferred);
>   		else
>   			zero_pages(obj_request->pages, xferred, length);
> -		obj_request->xferred = length;
>   	}
> +	obj_request->xferred = length;
>   	obj_request_done_set(obj_request);
>   }
>
>