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Re: [Qemu-block] [Qemu-devel] [PATCH 5/5] block: Move request_alignment

From: Kevin Wolf
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 5/5] block: Move request_alignment into BlockLimit
Date: Tue, 14 Jun 2016 10:05:46 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

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.


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