Copy snapshots out of QCOW2 disk

In order to backup snapshots, created from QCOW2 iamge, we want to copy snapshots out of QCOW2 disk to a seperate storage.
The following patch adds a new option in "qemu-img": qemu-img convert -f qcow2 -O qcow2 -s snapshot_name src_img bck_img.
Right now, it only supports to copy the full snapshot, delta snapshot is on the way.

Changes from V1: all the comments from Kevin are addressed:
Add read-only checking
Fix coding style
Change the name from bdrv_snapshot_load to bdrv_snapshot_load_tmp

Signed-off-by: Disheng Su <edison@cloud.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 31 insertions, 0 deletions

diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index 8dd5df0bcd..aacf357821 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -418,3 +418,34 @@ int qcow2_snapshot_list(BlockDriverState *bs, QEMUSnapshotInfo **psn_tab)
     return s->nb_snapshots;
 }
 
+int qcow2_snapshot_load_tmp(BlockDriverState *bs, const char *snapshot_name)
+{
+    int i, snapshot_index, l1_size2;
+    BDRVQcowState *s = bs->opaque;
+    QCowSnapshot *sn;
+
+    snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_name);
+    if (snapshot_index < 0) {
+        return -ENOENT;
+    }
+
+    sn = &s->snapshots[snapshot_index];
+    s->l1_size = sn->l1_size;
+    l1_size2 = s->l1_size * sizeof(uint64_t);
+    if (s->l1_table != NULL) {
+        qemu_free(s->l1_table);
+    }
+
+    s->l1_table_offset = sn->l1_table_offset;
+    s->l1_table = qemu_mallocz(align_offset(l1_size2, 512));
+
+    if (bdrv_pread(bs->file, sn->l1_table_offset,
+                   s->l1_table, l1_size2) != l1_size2) {
+        return -1;
+    }
+
+    for(i = 0;i < s->l1_size; i++) {
+        be64_to_cpus(&s->l1_table[i]);
+    }
+    return 0;
+}