[Qemu-devel] [PATCH 2/4] qcow: allow sub-cluster compressed write to last cluster

[Stefan Hajnoczi]

Compression in qcow requires image length to be a multiple of the
cluster size.  Lift this requirement by zero-padding the final cluster
when necessary.  The virtual disk size is still not cluster-aligned, so
the guest cannot access the zero sectors.

Note that this is almost identical to the qcow2 version of this code.
qcow2's compression code is drawn from qcow.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 block/qcow.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/block/qcow.c b/block/qcow.c
index 3278e55..e2a64c7 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -787,8 +787,21 @@ static int qcow_write_compressed(BlockDriverState *bs, 
int64_t sector_num,
     uint8_t *out_buf;
     uint64_t cluster_offset;
 
-    if (nb_sectors != s->cluster_sectors)
-        return -EINVAL;
+    if (nb_sectors != s->cluster_sectors) {
+        ret = -EINVAL;
+
+        /* Zero-pad last write if image size is not cluster aligned */
+        if (sector_num + nb_sectors == bs->total_sectors &&
+            nb_sectors < s->cluster_sectors) {
+            uint8_t *pad_buf = qemu_blockalign(bs, s->cluster_size);
+            memset(pad_buf, 0, s->cluster_size);
+            memcpy(pad_buf, buf, nb_sectors * BDRV_SECTOR_SIZE);
+            ret = qcow_write_compressed(bs, sector_num,
+                                        pad_buf, s->cluster_sectors);
+            qemu_vfree(pad_buf);
+        }
+        return ret;
+    }
 
     out_buf = g_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
 
-- 
1.8.1.4