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, 120 insertions, 0 deletions

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 8c17c0e3aa..88d5a3f1ad 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -29,6 +29,7 @@
 #include "block/qcow2.h"
 #include "qemu/range.h"
 #include "qemu/bswap.h"
+#include "qemu/cutils.h"
 
 static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size);
 static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
@@ -3061,3 +3062,122 @@ done:
     qemu_vfree(new_refblock);
     return ret;
 }
+
+static int qcow2_discard_refcount_block(BlockDriverState *bs,
+                                        uint64_t discard_block_offs)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t refblock_offs = get_refblock_offset(s, discard_block_offs);
+    uint64_t cluster_index = discard_block_offs >> s->cluster_bits;
+    uint32_t block_index = cluster_index & (s->refcount_block_size - 1);
+    void *refblock;
+    int ret;
+
+    assert(discard_block_offs != 0);
+
+    ret = qcow2_cache_get(bs, s->refcount_block_cache, refblock_offs,
+                          &refblock);
+    if (ret < 0) {
+        return ret;
+    }
+
+    if (s->get_refcount(refblock, block_index) != 1) {
+        qcow2_signal_corruption(bs, true, -1, -1, "Invalid refcount:"
+                                " refblock offset %#" PRIx64
+                                ", reftable index %u"
+                                ", block offset %#" PRIx64
+                                ", refcount %#" PRIx64,
+                                refblock_offs,
+                                offset_to_reftable_index(s, discard_block_offs),
+                                discard_block_offs,
+                                s->get_refcount(refblock, block_index));
+        qcow2_cache_put(bs, s->refcount_block_cache, &refblock);
+        return -EINVAL;
+    }
+    s->set_refcount(refblock, block_index, 0);
+
+    qcow2_cache_entry_mark_dirty(bs, s->refcount_block_cache, refblock);
+
+    qcow2_cache_put(bs, s->refcount_block_cache, &refblock);
+
+    if (cluster_index < s->free_cluster_index) {
+        s->free_cluster_index = cluster_index;
+    }
+
+    refblock = qcow2_cache_is_table_offset(bs, s->refcount_block_cache,
+                                           discard_block_offs);
+    if (refblock) {
+        /* discard refblock from the cache if refblock is cached */
+        qcow2_cache_discard(bs, s->refcount_block_cache, refblock);
+    }
+    update_refcount_discard(bs, discard_block_offs, s->cluster_size);
+
+    return 0;
+}
+
+int qcow2_shrink_reftable(BlockDriverState *bs)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t *reftable_tmp =
+        g_malloc(s->refcount_table_size * sizeof(uint64_t));
+    int i, ret;
+
+    for (i = 0; i < s->refcount_table_size; i++) {
+        int64_t refblock_offs = s->refcount_table[i] & REFT_OFFSET_MASK;
+        void *refblock;
+        bool unused_block;
+
+        if (refblock_offs == 0) {
+            reftable_tmp[i] = 0;
+            continue;
+        }
+        ret = qcow2_cache_get(bs, s->refcount_block_cache, refblock_offs,
+                              &refblock);
+        if (ret < 0) {
+            goto out;
+        }
+
+        /* the refblock has own reference */
+        if (i == offset_to_reftable_index(s, refblock_offs)) {
+            uint64_t block_index = (refblock_offs >> s->cluster_bits) &
+                                   (s->refcount_block_size - 1);
+            uint64_t refcount = s->get_refcount(refblock, block_index);
+
+            s->set_refcount(refblock, block_index, 0);
+
+            unused_block = buffer_is_zero(refblock, s->cluster_size);
+
+            s->set_refcount(refblock, block_index, refcount);
+        } else {
+            unused_block = buffer_is_zero(refblock, s->cluster_size);
+        }
+        qcow2_cache_put(bs, s->refcount_block_cache, &refblock);
+
+        reftable_tmp[i] = unused_block ? 0 : cpu_to_be64(s->refcount_table[i]);
+    }
+
+    ret = bdrv_pwrite_sync(bs->file, s->refcount_table_offset, reftable_tmp,
+                           s->refcount_table_size * sizeof(uint64_t));
+    /*
+     * If the write in the reftable failed the image may contain a partially
+     * overwritten reftable. In this case it would be better to clear the
+     * reftable in memory to avoid possible image corruption.
+     */
+    for (i = 0; i < s->refcount_table_size; i++) {
+        if (s->refcount_table[i] && !reftable_tmp[i]) {
+            if (ret == 0) {
+                ret = qcow2_discard_refcount_block(bs, s->refcount_table[i] &
+                                                       REFT_OFFSET_MASK);
+            }
+            s->refcount_table[i] = 0;
+        }
+    }
+
+    if (!s->cache_discards) {
+        qcow2_process_discards(bs, ret);
+    }
+
+out:
+    g_free(reftable_tmp);
+    return ret;
+}