[Qemu-devel] [PATCH 2/2] qcow2: truncate the tail of the image file after shrinking the image

[Pavel Butsykin <pbutsykin@virtuozzo.com>]

Now after shrinking the image, at the end of the image file, there might be a
tail that probably will never be used. So we can find the last used cluster and
cut the tail.

Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
---
 block/qcow2-refcount.c | 21 +++++++++++++++++++++
 block/qcow2.c          | 19 +++++++++++++++++++
 block/qcow2.h          |  1 +
 3 files changed, 41 insertions(+)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 88d5a3f1ad..5e221a166c 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -3181,3 +3181,24 @@ out:
     g_free(reftable_tmp);
     return ret;
 }
+
+int64_t qcow2_get_last_cluster(BlockDriverState *bs, int64_t size)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int64_t i, last_cluster, nb_clusters = size_to_clusters(s, size);
+    uint64_t refcount;
+
+    for (i = 0, last_cluster = 0; i < nb_clusters; i++) {
+        int ret = qcow2_get_refcount(bs, i, &refcount);
+        if (ret < 0) {
+            fprintf(stderr, "Can't get refcount for cluster %" PRId64 ": %s\n",
+                    i, strerror(-ret));
+            continue;
+        }
+
+        if (refcount > 0) {
+            last_cluster = i;
+        }
+    }
+    return last_cluster;
+}
diff --git a/block/qcow2.c b/block/qcow2.c
index 8a4311d338..c3b6dd44c4 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -3106,6 +3106,7 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
     new_l1_size = size_to_l1(s, offset);
 
     if (offset < old_length) {
+        int64_t image_end_offset, old_file_size;
         if (prealloc != PREALLOC_MODE_OFF) {
             error_setg(errp,
                        "Preallocation can't be used for shrinking an image");
@@ -3134,6 +3135,24 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
                              "Failed to discard unused refblocks");
             return ret;
         }
+
+        old_file_size = bdrv_getlength(bs->file->bs);
+        if (old_file_size < 0) {
+            error_setg_errno(errp, -old_file_size,
+                             "Failed to inquire current file length");
+            return old_file_size;
+        }
+        image_end_offset = (qcow2_get_last_cluster(bs, old_file_size) + 1) *
+                           s->cluster_size;
+        if (image_end_offset < old_file_size) {
+            ret = bdrv_truncate(bs->file, image_end_offset,
+                                PREALLOC_MODE_OFF, NULL);
+            if (ret < 0) {
+                error_setg_errno(errp, -ret,
+                                 "Failed to truncate the tail of the image");
+                return ret;
+            }
+        }
     } else {
         ret = qcow2_grow_l1_table(bs, new_l1_size, true);
         if (ret < 0) {
diff --git a/block/qcow2.h b/block/qcow2.h
index 5a289a81e2..782a206ecb 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -597,6 +597,7 @@ int qcow2_change_refcount_order(BlockDriverState *bs, int refcount_order,
                                 BlockDriverAmendStatusCB *status_cb,
                                 void *cb_opaque, Error **errp);
 int qcow2_shrink_reftable(BlockDriverState *bs);
+int64_t qcow2_get_last_cluster(BlockDriverState *bs, int64_t size);
 
 /* qcow2-cluster.c functions */
 int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
-- 
2.14.1