commit a3d6ae2299eaab1bced05551d0a0abfbcd9d08d0
author Nir Soffer <nirsof@gmail.com> Sun Mar 24 02:20:12 2019 +0200
committer Kevin Wolf <kwolf@redhat.com> Tue Apr 02 12:04:44 2019 +0200
tree 22b211b539ad675b5da52fd5f5cb57ee3b0bbb78
parent e0a59749efc246646bb208e553489b894450cbcd

qemu-img: Enable BDRV_REQ_MAY_UNMAP in convert

With Kevin's "block: Fix slow pre-zeroing in qemu-img convert"[1]
(commit c9fdcf202f, 'qemu-img: Use BDRV_REQ_NO_FALLBACK for
pre-zeroing') we skip the pre zero step called like this:

    blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK)

And we write zeroes later using:

    blk_co_pwrite_zeroes(s->target,
                         sector_num << BDRV_SECTOR_BITS,
                         n << BDRV_SECTOR_BITS, 0);

Since we use flags=0, this is translated to NBD_CMD_WRITE_ZEROES with
NBD_CMD_FLAG_NO_HOLE flag, which cause the NBD server to allocated space
instead of punching a hole.

Here is an example failure:

$ dd if=/dev/urandom of=src.img bs=1M count=5
$ truncate -s 50m src.img
$ truncate -s 50m dst.img
$ nbdkit -f -v -e '' -U nbd.sock file file=dst.img

$ ./qemu-img convert -n src.img nbd:unix:nbd.sock

We can see in nbdkit log that it received the NBD_CMD_FLAG_NO_HOLE
(may_trim=0):

nbdkit: file[1]: debug: newstyle negotiation: flags: export 0x4d
nbdkit: file[1]: debug: pwrite count=2097152 offset=0
nbdkit: file[1]: debug: pwrite count=2097152 offset=2097152
nbdkit: file[1]: debug: pwrite count=1048576 offset=4194304
nbdkit: file[1]: debug: zero count=33554432 offset=5242880 may_trim=0
nbdkit: file[1]: debug: zero count=13631488 offset=38797312 may_trim=0
nbdkit: file[1]: debug: flush

And the image became fully allocated:

$ qemu-img info dst.img
virtual size: 50M (52428800 bytes)
disk size: 50M

With this change we see that nbdkit did not receive the
NBD_CMD_FLAG_NO_HOLE (may_trim=1):

nbdkit: file[1]: debug: newstyle negotiation: flags: export 0x4d
nbdkit: file[1]: debug: pwrite count=2097152 offset=0
nbdkit: file[1]: debug: pwrite count=2097152 offset=2097152
nbdkit: file[1]: debug: pwrite count=1048576 offset=4194304
nbdkit: file[1]: debug: zero count=33554432 offset=5242880 may_trim=1
nbdkit: file[1]: debug: zero count=13631488 offset=38797312 may_trim=1
nbdkit: file[1]: debug: flush

And the file is sparse as expected:

$ qemu-img info dst.img
virtual size: 50M (52428800 bytes)
disk size: 5.0M

[1] http://lists.nongnu.org/archive/html/qemu-block/2019-03/msg00761.html

Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

diff --git a/qemu-img.c b/qemu-img.c
index 76a961d..aa6f81f 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1756,7 +1756,8 @@
             }
             ret = blk_co_pwrite_zeroes(s->target,
                                        sector_num << BDRV_SECTOR_BITS,
-                                       n << BDRV_SECTOR_BITS, 0);
+                                       n << BDRV_SECTOR_BITS,
+                                       BDRV_REQ_MAY_UNMAP);
             if (ret < 0) {
                 return ret;
             }