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authorMax Reitz <mreitz@redhat.com>2014-10-24 15:57:32 +0200
committerStefan Hajnoczi <stefanha@redhat.com>2014-11-03 11:41:48 +0000
commit94054183daffaa41cd77ced9301c01a01027923a (patch)
tree75c6d4efe55d181cee75d7e3eaaf59ecbf1e9aa5 /block/qcow2.c
parent491d27e2af4f6e157c4b29d43269c5cb0d191171 (diff)
qcow2: Optimize bdrv_make_empty()
bdrv_make_empty() is currently only called if the current image represents an external snapshot that has been committed to its base image; it is therefore unlikely to have internal snapshots. In this case, bdrv_make_empty() can be greatly sped up by emptying the L1 and refcount table (while having the dirty flag set, which only works for compat=1.1) and creating a trivial refcount structure. If there are snapshots or for compat=0.10, fall back to the simple implementation (discard all clusters). [Applied s/clusters/cluster/ typo fix suggested by Eric Blake --Stefan] Signed-off-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Message-id: 1414159063-25977-4-git-send-email-mreitz@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Diffstat (limited to 'block/qcow2.c')
-rw-r--r--block/qcow2.c165
1 files changed, 164 insertions, 1 deletions
diff --git a/block/qcow2.c b/block/qcow2.c
index bf871d58f4..7ec7830fac 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2230,12 +2230,175 @@ fail:
return ret;
}
+static int make_completely_empty(BlockDriverState *bs)
+{
+ BDRVQcowState *s = bs->opaque;
+ int ret, l1_clusters;
+ int64_t offset;
+ uint64_t *new_reftable = NULL;
+ uint64_t rt_entry, l1_size2;
+ struct {
+ uint64_t l1_offset;
+ uint64_t reftable_offset;
+ uint32_t reftable_clusters;
+ } QEMU_PACKED l1_ofs_rt_ofs_cls;
+
+ ret = qcow2_cache_empty(bs, s->l2_table_cache);
+ if (ret < 0) {
+ goto fail;
+ }
+
+ ret = qcow2_cache_empty(bs, s->refcount_block_cache);
+ if (ret < 0) {
+ goto fail;
+ }
+
+ /* Refcounts will be broken utterly */
+ ret = qcow2_mark_dirty(bs);
+ if (ret < 0) {
+ goto fail;
+ }
+
+ BLKDBG_EVENT(bs->file, BLKDBG_L1_UPDATE);
+
+ l1_clusters = DIV_ROUND_UP(s->l1_size, s->cluster_size / sizeof(uint64_t));
+ l1_size2 = (uint64_t)s->l1_size * sizeof(uint64_t);
+
+ /* After this call, neither the in-memory nor the on-disk refcount
+ * information accurately describe the actual references */
+
+ ret = bdrv_write_zeroes(bs->file, s->l1_table_offset / BDRV_SECTOR_SIZE,
+ l1_clusters * s->cluster_sectors, 0);
+ if (ret < 0) {
+ goto fail_broken_refcounts;
+ }
+ memset(s->l1_table, 0, l1_size2);
+
+ BLKDBG_EVENT(bs->file, BLKDBG_EMPTY_IMAGE_PREPARE);
+
+ /* Overwrite enough clusters at the beginning of the sectors to place
+ * the refcount table, a refcount block and the L1 table in; this may
+ * overwrite parts of the existing refcount and L1 table, which is not
+ * an issue because the dirty flag is set, complete data loss is in fact
+ * desired and partial data loss is consequently fine as well */
+ ret = bdrv_write_zeroes(bs->file, s->cluster_size / BDRV_SECTOR_SIZE,
+ (2 + l1_clusters) * s->cluster_size /
+ BDRV_SECTOR_SIZE, 0);
+ /* This call (even if it failed overall) may have overwritten on-disk
+ * refcount structures; in that case, the in-memory refcount information
+ * will probably differ from the on-disk information which makes the BDS
+ * unusable */
+ if (ret < 0) {
+ goto fail_broken_refcounts;
+ }
+
+ BLKDBG_EVENT(bs->file, BLKDBG_L1_UPDATE);
+ BLKDBG_EVENT(bs->file, BLKDBG_REFTABLE_UPDATE);
+
+ /* "Create" an empty reftable (one cluster) directly after the image
+ * header and an empty L1 table three clusters after the image header;
+ * the cluster between those two will be used as the first refblock */
+ cpu_to_be64w(&l1_ofs_rt_ofs_cls.l1_offset, 3 * s->cluster_size);
+ cpu_to_be64w(&l1_ofs_rt_ofs_cls.reftable_offset, s->cluster_size);
+ cpu_to_be32w(&l1_ofs_rt_ofs_cls.reftable_clusters, 1);
+ ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, l1_table_offset),
+ &l1_ofs_rt_ofs_cls, sizeof(l1_ofs_rt_ofs_cls));
+ if (ret < 0) {
+ goto fail_broken_refcounts;
+ }
+
+ s->l1_table_offset = 3 * s->cluster_size;
+
+ new_reftable = g_try_new0(uint64_t, s->cluster_size / sizeof(uint64_t));
+ if (!new_reftable) {
+ ret = -ENOMEM;
+ goto fail_broken_refcounts;
+ }
+
+ s->refcount_table_offset = s->cluster_size;
+ s->refcount_table_size = s->cluster_size / sizeof(uint64_t);
+
+ g_free(s->refcount_table);
+ s->refcount_table = new_reftable;
+ new_reftable = NULL;
+
+ /* Now the in-memory refcount information again corresponds to the on-disk
+ * information (reftable is empty and no refblocks (the refblock cache is
+ * empty)); however, this means some clusters (e.g. the image header) are
+ * referenced, but not refcounted, but the normal qcow2 code assumes that
+ * the in-memory information is always correct */
+
+ BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC);
+
+ /* Enter the first refblock into the reftable */
+ rt_entry = cpu_to_be64(2 * s->cluster_size);
+ ret = bdrv_pwrite_sync(bs->file, s->cluster_size,
+ &rt_entry, sizeof(rt_entry));
+ if (ret < 0) {
+ goto fail_broken_refcounts;
+ }
+ s->refcount_table[0] = 2 * s->cluster_size;
+
+ s->free_cluster_index = 0;
+ assert(3 + l1_clusters <= s->refcount_block_size);
+ offset = qcow2_alloc_clusters(bs, 3 * s->cluster_size + l1_size2);
+ if (offset < 0) {
+ ret = offset;
+ goto fail_broken_refcounts;
+ } else if (offset > 0) {
+ error_report("First cluster in emptied image is in use");
+ abort();
+ }
+
+ /* Now finally the in-memory information corresponds to the on-disk
+ * structures and is correct */
+ ret = qcow2_mark_clean(bs);
+ if (ret < 0) {
+ goto fail;
+ }
+
+ ret = bdrv_truncate(bs->file, (3 + l1_clusters) * s->cluster_size);
+ if (ret < 0) {
+ goto fail;
+ }
+
+ return 0;
+
+fail_broken_refcounts:
+ /* The BDS is unusable at this point. If we wanted to make it usable, we
+ * would have to call qcow2_refcount_close(), qcow2_refcount_init(),
+ * qcow2_check_refcounts(), qcow2_refcount_close() and qcow2_refcount_init()
+ * again. However, because the functions which could have caused this error
+ * path to be taken are used by those functions as well, it's very likely
+ * that that sequence will fail as well. Therefore, just eject the BDS. */
+ bs->drv = NULL;
+
+fail:
+ g_free(new_reftable);
+ return ret;
+}
+
static int qcow2_make_empty(BlockDriverState *bs)
{
- int ret = 0;
+ BDRVQcowState *s = bs->opaque;
uint64_t start_sector;
int sector_step = INT_MAX / BDRV_SECTOR_SIZE;
+ int l1_clusters, ret = 0;
+
+ l1_clusters = DIV_ROUND_UP(s->l1_size, s->cluster_size / sizeof(uint64_t));
+
+ if (s->qcow_version >= 3 && !s->snapshots &&
+ 3 + l1_clusters <= s->refcount_block_size) {
+ /* The following function only works for qcow2 v3 images (it requires
+ * the dirty flag) and only as long as there are no snapshots (because
+ * it completely empties the image). Furthermore, the L1 table and three
+ * additional clusters (image header, refcount table, one refcount
+ * block) have to fit inside one refcount block. */
+ return make_completely_empty(bs);
+ }
+ /* This fallback code simply discards every active cluster; this is slow,
+ * but works in all cases */
for (start_sector = 0; start_sector < bs->total_sectors;
start_sector += sector_step)
{