From c018040299805135af45098641391a2818ddac8a Mon Sep 17 00:00:00 2001

From: Maxim Levitsky <mlevitsk@redhat.com>

Date: Mon, 26 Aug 2019 09:19:44 +0200

Subject: [PATCH 4/4] qemu-img: Enable BDRV_REQ_MAY_UNMAP in convert

RH-Author: Maxim Levitsky <mlevitsk@redhat.com>

Message-id: <20190826091944.16113-2-mlevitsk@redhat.com>

Patchwork-id: 90164

O-Subject: [RHEL-7.7.z qemu-kvm-rhev PATCH v3 1/1] qemu-img: Enable BDRV_REQ_MAY_UNMAP in convert

Bugzilla: 1648622

RH-Acked-by: John Snow <jsnow@redhat.com>

RH-Acked-by: Max Reitz <mreitz@redhat.com>

RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>

From: Nir Soffer <nirsof@gmail.com>

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>

(cherry picked from commit a3d6ae2299eaab1bced05551d0a0abfbcd9d08d0)

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

---

 qemu-img.c | 3 ++-

 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/qemu-img.c b/qemu-img.c

index d588183..f7c1483 100644

--- a/qemu-img.c

+++ b/qemu-img.c

@@ -1738,7 +1738,8 @@ static int coroutine_fn convert_co_write(ImgConvertState *s, int64_t sector_num,

             }

             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;

             }

-- 

1.8.3.1