Am 14.06.2016 um 16:47 hat Eric Blake geschrieben: > On 06/14/2016 02:05 AM, Kevin Wolf wrote: > > >>>> 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. > > Or maybe split the main bdrv_refresh_limits() into two pieces: one that > merges limits from one BDS into another (the limits that are worth > merging, and in the correct direction: opt merges to MAX, max merges to > MIN_NON_ZERO, request_alignment is not merged), the other that calls > merge(bs, bs->file->bs); then have raw_refresh_limits() also use the > common merge functionality rather than straight copy. Testing that > approach now...
So you don't agree with what I said above? I think that block drivers should only set limits that they require for themselves. The block layer bdrv_refresh_limits() function can then merge things where needed; drivers shouldn't be involved there. Kevin
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