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

From: Vladimir Sementsov-Ogievskiy
Subject: [Qemu-block] [PATCH] backup: allow target without .bdrv_get_info
Date: Tue, 14 Feb 2017 17:48:24 +0300

Currently backup to nbd target is broken, as nbd doesn't have
.bdrv_get_info realization.

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

Hi all!

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

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 | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/backup.c b/block/backup.c
index ea38733..095a390 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -638,7 +638,10 @@ 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 */
+        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");

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