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Re: [Qemu-block] [PATCH v2] backup: allow target without .bdrv_get_info

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH v2] backup: allow target without .bdrv_get_info
Date: Wed, 15 Feb 2017 14:42:34 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.1

15.02.2017 14:33, Fam Zheng wrote:
On Wed, 02/15 14:09, Vladimir Sementsov-Ogievskiy wrote:
Currently backup to nbd target is broken, as nbd doesn't have
.bdrv_get_info realization.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>

v2: add WARNING


Since commit

commit 4c9bca7e39a6e07ad02c1dcde3478363344ec60b
Author: John Snow <address@hidden>
Date:   Thu Feb 25 15:58:30 2016 -0500

     block/backup: avoid copying less than full target clusters

backup to nbd target is broken, we have "Couldn't determine the cluster size of
the target image".

Proposed NBD protocol extension - NBD_OPT_INFO should finally solve this 
But until it is not realized, we need allow backup to nbd target due to backward
If you respin, including above in the commit message is even better, IMO.

Furthermore, is it entirely ok to disallow backup if bds lacks .bdrv_get_info?
Which behavior should be default: to fail backup or to use default cluster size?

  block/backup.c | 10 +++++++++-
  1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/block/backup.c b/block/backup.c
index ea38733..a464b63 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -638,7 +638,15 @@ BlockJob *backup_job_create(const char *job_id, 
BlockDriverState *bs,
       * backup cluster size is smaller than the target cluster size. Even for
       * targets with a backing file, try to avoid COW if possible. */
      ret = bdrv_get_info(target, &bdi);
-    if (ret < 0 && !target->backing) {
+    if (ret == -ENOTSUP) {
+        /* Cluster size is not defined */
+        error_report("WARNING: Target block device doesn't provide information 
+                     "about block size and it doesn't have backing file. "
+                     "Default block size of %u bytes is used. If actual block "
+                     "size of target exceeds this default, destination image "
+                     "may become unusable");
"Image unusable" might be too strong here because it sounds like a serious
damage to the image, while actually what we mean is "backup unusable".

Finally, "... default, backup may be unusable"?

+        job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
+    } else if (ret < 0 && !target->backing) {
          error_setg_errno(errp, -ret,
              "Couldn't determine the cluster size of the target image, "
              "which has no backing file");


Best regards,

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