Am 14.06.2016 um 06:39 hat Eric Blake geschrieben: > On 06/07/2016 04:08 AM, Kevin Wolf wrote: > > >> Found it; squash this in (or use it as an argument why we don't want > >> request_alignment in bs->bl after all): > > > > This hunk doesn't make sense to me. For the correctness of the code it > > shouldn't make a difference whether the alignment happens before passing > > the request to file/raw-posix or already in the raw format layer. > > > > The cause for the hang you're seeing is probably that the request is > > already aligned before the blkdebug layer and therefore the blkdebug > > events aren't generated any more. That's a problem with the test (I'm > > considering the blkdebug events part of the test infrastructure), > > however, and not with the code. > > > > Yes, it's definitely a hang caused by the test expecting an unalignment > event, but the inheritance chain now causes things to be aligned to > begin with and nothing unaligned happens after all. > > > Kevin > > > >> diff --git i/block/raw_bsd.c w/block/raw_bsd.c > >> index b1d5237..c3c2246 100644 > >> --- i/block/raw_bsd.c > >> +++ w/block/raw_bsd.c > >> @@ -152,7 +152,11 @@ static int raw_get_info(BlockDriverState *bs, > >> BlockDriverInfo *bdi) > >> > >> static void raw_refresh_limits(BlockDriverState *bs, Error **errp) > >> { > >> + /* Inherit all limits except for request_alignment */ > >> + int request_alignment = bs->bl.request_alignment; > >> + > >> bs->bl = bs->file->bs->bl; > >> + bs->bl.request_alignment = request_alignment; > > Any ideas on how to fix the test, then? Have two blkdebug devices > nested atop one another, since those are the devices where we can > explicitly override alignment?
Interesting idea. Maybe that's a good option if it works. > (normally, you'd _expect_ the chain to > inherit the worst-case alignment of all BDS in the chain, so blkdebug is > the way around it). Actually, I think there are two cases. The first one is that you get a request and want to know what to do with it. Here you don't want to inherit the worst-case alignment. You'd rather want to enforce alignment only when it is actually needed. For example, if you have a cache=none backing file with 4k alignment, but a cache=writeback overlay, and you don't actually access the backing file with this request, it would be wasteful to align the request. You also don't really know that a driver will issue a misaligned request (or any request at all) to the lower layer just because it got called with one. The second case is when you want to issue a request. For example, let's take the qcow2 cache here, which has 64k cached and needs to update a few bytes. Currently it always writes the full 64k (and with 1 MB clusters a full megabyte), but what it really should do is consider the worst-case alignment. This is comparable to the difference between bdrv_opt_mem_align(), which is used in qemu_blockalign() where we want to create a buffer that works even in the worst case, and bdrv_min_mem_align(), which is used in bdrv_qiov_is_aligned() in order to determine whether we must align an existing request. > That's the only thing left before I repost the series, so I may just > post the last patch as RFC and play with it a bit more... And in the light of the above, maybe the solution is as easy as changing raw_refresh_limits() to set bs->bl.request_alignment = 1. Kevin
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