block-copy: move detecting fleecing scheme to block-copy

We want to simplify initialization interface of copy-before-write
filter as we are going to make it public. So, let's detect fleecing
scheme exactly in block-copy code, to not pass this information through
extra levels.

Why not just set BDRV_REQ_SERIALISING unconditionally: because we are
going to implement new more efficient fleecing scheme which will not
rely on backing feature.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20210824083856.17408-7-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

1 files changed, 1 insertions, 20 deletions

diff --git a/block/backup.c b/block/backup.c
index ac91821b08..84f9a5f02c 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -407,7 +407,6 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
     int64_t len, target_len;
     BackupBlockJob *job = NULL;
     int64_t cluster_size;
-    BdrvRequestFlags write_flags;
     BlockDriverState *cbw = NULL;
     BlockCopyState *bcs = NULL;
 
@@ -504,26 +503,8 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
         goto error;
     }
 
-    /*
-     * If source is in backing chain of target assume that target is going to be
-     * used for "image fleecing", i.e. it should represent a kind of snapshot of
-     * source at backup-start point in time. And target is going to be read by
-     * somebody (for example, used as NBD export) during backup job.
-     *
-     * In this case, we need to add BDRV_REQ_SERIALISING write flag to avoid
-     * intersection of backup writes and third party reads from target,
-     * otherwise reading from target we may occasionally read already updated by
-     * guest data.
-     *
-     * For more information see commit f8d59dfb40bb and test
-     * tests/qemu-iotests/222
-     */
-    write_flags = (bdrv_chain_contains(target, bs) ? BDRV_REQ_SERIALISING : 0) |
-                  (compress ? BDRV_REQ_WRITE_COMPRESSED : 0),
-
     cbw = bdrv_cbw_append(bs, target, filter_node_name,
-                                        cluster_size, perf,
-                                        write_flags, &bcs, errp);
+                          cluster_size, perf, compress, &bcs, errp);
     if (!cbw) {
         goto error;
     }