qcow2: Implement bdrv_truncate() for growing images

This patch adds the ability to grow qcow2 images in-place using
bdrv_truncate().  This enables qemu-img resize command support for
qcow2.

Snapshots are not supported and bdrv_truncate() will return -ENOTSUP.
The notion of resizing an image with snapshots could lead to confusion:
users may expect snapshots to remain unchanged, but this is not possible
with the current qcow2 on-disk format where the header.size field is
global instead of per-snapshot.  Others may expect snapshots to change
size along with the current image data.  I think it is safest to not
support snapshots and perhaps add behavior later if there is a
consensus.

Backing images continue to work.  If the image is now larger than its
backing image, zeroes are read when accessing beyond the end of the
backing image.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -140,7 +140,7 @@ static int qcow_read_extensions(BlockDriverState *bs, uint64_t start_offset,
 static int qcow_open(BlockDriverState *bs, int flags)
 {
     BDRVQcowState *s = bs->opaque;
-    int len, i, shift;
+    int len, i;
     QCowHeader header;
     uint64_t ext_end;
 
@@ -188,8 +188,7 @@ static int qcow_open(BlockDriverState *bs, int flags)
 
     /* read the level 1 table */
     s->l1_size = header.l1_size;
-    shift = s->cluster_bits + s->l2_bits;
-    s->l1_vm_state_index = (header.size + (1LL << shift) - 1) >> shift;
+    s->l1_vm_state_index = size_to_l1(s, header.size);
     /* the L1 table must contain at least enough entries to put
        header.size bytes */
     if (s->l1_size < s->l1_vm_state_index)
@@ -851,6 +850,43 @@ static int qcow_make_empty(BlockDriverState *bs)
     return 0;
 }
 
+static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret, new_l1_size;
+
+    if (offset & 511) {
+        return -EINVAL;
+    }
+
+    /* cannot proceed if image has snapshots */
+    if (s->nb_snapshots) {
+        return -ENOTSUP;
+    }
+
+    /* shrinking is currently not supported */
+    if (offset < bs->total_sectors * 512) {
+        return -ENOTSUP;
+    }
+
+    new_l1_size = size_to_l1(s, offset);
+    ret = qcow2_grow_l1_table(bs, new_l1_size);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* write updated header.size */
+    offset = cpu_to_be64(offset);
+    ret = bdrv_pwrite(bs->file, offsetof(QCowHeader, size),
+                      &offset, sizeof(uint64_t));
+    if (ret < 0) {
+        return ret;
+    }
+
+    s->l1_vm_state_index = new_l1_size;
+    return 0;
+}
+
 /* XXX: put compressed sectors first, then all the cluster aligned
    tables to avoid losing bytes in alignment */
 static int qcow_write_compressed(BlockDriverState *bs, int64_t sector_num,
@@ -1050,6 +1086,8 @@ static BlockDriver bdrv_qcow2 = {
     .bdrv_aio_readv	= qcow_aio_readv,
     .bdrv_aio_writev	= qcow_aio_writev,
     .bdrv_aio_flush	= qcow_aio_flush,
+
+    .bdrv_truncate          = qcow2_truncate,
     .bdrv_write_compressed  = qcow_write_compressed,
 
     .bdrv_snapshot_create   = qcow2_snapshot_create,

block/qcow2.h

@@ -150,6 +150,12 @@ static inline int size_to_clusters(BDRVQcowState *s, int64_t size)
     return (size + (s->cluster_size - 1)) >> s->cluster_bits;
 }
 
+static inline int size_to_l1(BDRVQcowState *s, int64_t size)
+{
+    int shift = s->cluster_bits + s->l2_bits;
+    return (size + (1ULL << shift) - 1) >> shift;
+}
+
 static inline int64_t align_offset(int64_t offset, int n)
 {
     offset = (offset + n - 1) & ~(n - 1);