Re: [PATCH] qemu-img map: Don't limit block status request size

[Eric Blake]

On 7/7/20 10:21 AM, Kevin Wolf wrote:
> Am 07.07.2020 um 16:54 hat Eric Blake geschrieben:
> > On 7/7/20 9:46 AM, Kevin Wolf wrote:
> > > Limiting each loop iteration of qemu-img map to 1 GB was arbitrary from
> > > the beginning, though it only cut the maximum in half then because the
> > > interface a signed 32 bit byte count. These days, bdrv_block_status()

interface was a

> > > supports a 64 bit byte count, so the arbitrary limit is even worse.
> > >
> > > On file-posix, bdrv_block_status() eventually maps to SEEK_HOLE and
> > > SEEK_DATA, which don't support a limit, but always do all of the work
> > > necessary to find the start of the next hole/data. Much of this work may
> > > be repeated if we don't use this information fully, but query with an
> > > only slightly larger offset in the next loop iteration. Therefore, if
> > > bdrv_block_status() is called in a loop, it should always pass the
> > > full number of bytes that the whole loop is interested in.
> > >
> > > This removes the arbitrary limit and speeds up 'qemu-img map'
> > > significantly on heavily fragmented images.
> > >
> > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > > ---
> > >    qemu-img.c | 5 +----
> > >    1 file changed, 1 insertion(+), 4 deletions(-)
> >
> > Do you want this in 5.1?  It seems like a nice optimization.
>
> I guess now that I have your R-b, I can sneak both patches in for soft
> freeze. :-)

Can we treat optimizations for speed problems as bug fixes if they land 
after soft freeze but before -rc1?

At any rate, this post reminds me that Vladimir's series to support 
64-bit actions elsewhere has probably slipped into 5.2 territory, but I 
still need to fix the fact that nbd is sending uint32_t trim/zero values 
into signed int block layer functions (Vladimir's work is nicer but 
harder to review, so I'll probably end up writing one-off patches for 
5.1 with minimal churn to just block/nbd.c rather than the whole block 
layer...)

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org