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>

block/blkdebug.c

@@ -195,6 +195,8 @@ static const char *event_names[BLKDBG_EVENT_MAX] = {
     [BLKDBG_PWRITEV]                        = "pwritev",
     [BLKDBG_PWRITEV_ZERO]                   = "pwritev_zero",
     [BLKDBG_PWRITEV_DONE]                   = "pwritev_done",
+
+    [BLKDBG_EMPTY_IMAGE_PREPARE]            = "empty_image_prepare",
 };
 
 static int get_event_by_name(const char *name, BlkDebugEvent *event)

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)
     {

include/block/block.h

@@ -498,6 +498,8 @@ typedef enum {
     BLKDBG_PWRITEV_ZERO,
     BLKDBG_PWRITEV_DONE,
 
+    BLKDBG_EMPTY_IMAGE_PREPARE,
+
     BLKDBG_EVENT_MAX,
 } BlkDebugEvent;