qcow2: add shrink image support

This patch add shrinking of the image file for qcow2. As a result, this allows
us to reduce the virtual image size and free up space on the disk without
copying the image. Image can be fragmented and shrink is done by punching holes
in the image file.

Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20170918124230.8152-4-pbutsykin@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 50 insertions, 0 deletions

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 0d4824993c..d2518d1893 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -32,6 +32,56 @@
 #include "qemu/bswap.h"
 #include "trace.h"
 
+int qcow2_shrink_l1_table(BlockDriverState *bs, uint64_t exact_size)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int new_l1_size, i, ret;
+
+    if (exact_size >= s->l1_size) {
+        return 0;
+    }
+
+    new_l1_size = exact_size;
+
+#ifdef DEBUG_ALLOC2
+    fprintf(stderr, "shrink l1_table from %d to %d\n", s->l1_size, new_l1_size);
+#endif
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_WRITE_TABLE);
+    ret = bdrv_pwrite_zeroes(bs->file, s->l1_table_offset +
+                                       new_l1_size * sizeof(uint64_t),
+                             (s->l1_size - new_l1_size) * sizeof(uint64_t), 0);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    ret = bdrv_flush(bs->file->bs);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_FREE_L2_CLUSTERS);
+    for (i = s->l1_size - 1; i > new_l1_size - 1; i--) {
+        if ((s->l1_table[i] & L1E_OFFSET_MASK) == 0) {
+            continue;
+        }
+        qcow2_free_clusters(bs, s->l1_table[i] & L1E_OFFSET_MASK,
+                            s->cluster_size, QCOW2_DISCARD_ALWAYS);
+        s->l1_table[i] = 0;
+    }
+    return 0;
+
+fail:
+    /*
+     * If the write in the l1_table failed the image may contain a partially
+     * overwritten l1_table. In this case it would be better to clear the
+     * l1_table in memory to avoid possible image corruption.
+     */
+    memset(s->l1_table + new_l1_size, 0,
+           (s->l1_size - new_l1_size) * sizeof(uint64_t));
+    return ret;
+}
+
 int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
                         bool exact_size)
 {