[Qemu-devel] [PATCH 7/8] block/qcow2: Speed up zero cluster expansion

[Max Reitz]

Actually, we do not need to allocate a new data cluster for every zero
cluster to be expanded: It is completely sufficient to rely on qcow2's
COW part and instead create a single zero cluster and reuse it as much
as possible.

Signed-off-by: Max Reitz <address@hidden>
---
 block/qcow2-cluster.c | 119 ++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 92 insertions(+), 27 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 905beb6..867db03 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1558,6 +1558,9 @@ static int expand_zero_clusters_in_l1(BlockDriverState 
*bs, uint64_t *l1_table,
     BDRVQcowState *s = bs->opaque;
     bool is_active_l1 = (l1_table == s->l1_table);
     uint64_t *l2_table = NULL;
+    int64_t zeroed_cluster_offset = 0;
+    int zeroed_cluster_refcount = 0;
+    int last_zeroed_cluster_l1i = 0, last_zeroed_cluster_l2i = 0;
     int ret;
     int i, j;
 
@@ -1617,47 +1620,79 @@ static int expand_zero_clusters_in_l1(BlockDriverState 
*bs, uint64_t *l1_table,
                     continue;
                 }
 
-                offset = qcow2_alloc_clusters(bs, s->cluster_size);
-                if (offset < 0) {
-                    ret = offset;
-                    goto fail;
+                if (zeroed_cluster_offset) {
+                    zeroed_cluster_refcount += l2_refcount;
+                    if (zeroed_cluster_refcount > 0xffff) {
+                        zeroed_cluster_refcount = 0;
+                        zeroed_cluster_offset = 0;
+                    }
                 }
+                if (!zeroed_cluster_offset) {
+                    offset = qcow2_alloc_clusters(bs, s->cluster_size);
+                    if (offset < 0) {
+                        ret = offset;
+                        goto fail;
+                    }
 
-                if (l2_refcount > 1) {
-                    /* For shared L2 tables, set the refcount accordingly (it 
is
-                     * already 1 and needs to be l2_refcount) */
-                    ret = qcow2_update_cluster_refcount(bs,
-                            offset >> s->cluster_bits, l2_refcount - 1,
-                            QCOW2_DISCARD_OTHER);
+                    ret = qcow2_pre_write_overlap_check(bs, 0, offset,
+                                                        s->cluster_size);
+                    if (ret < 0) {
+                        qcow2_free_clusters(bs, offset, s->cluster_size,
+                                            QCOW2_DISCARD_OTHER);
+                        goto fail;
+                    }
+
+                    ret = bdrv_write_zeroes(bs->file, offset / 
BDRV_SECTOR_SIZE,
+                                            s->cluster_sectors, 0);
                     if (ret < 0) {
                         qcow2_free_clusters(bs, offset, s->cluster_size,
                                             QCOW2_DISCARD_OTHER);
                         goto fail;
                     }
+
+                    if (l2_refcount > 1) {
+                        ret = qcow2_update_cluster_refcount(bs,
+                                offset >> s->cluster_bits, l2_refcount - 1,
+                                QCOW2_DISCARD_OTHER);
+                        if (ret < 0) {
+                            qcow2_free_clusters(bs, offset, s->cluster_size,
+                                                QCOW2_DISCARD_OTHER);
+                            goto fail;
+                        }
+                    }
+
+                    zeroed_cluster_offset = offset;
+                    zeroed_cluster_refcount = l2_refcount;
+                } else {
+                    ret = qcow2_update_cluster_refcount(bs,
+                            zeroed_cluster_offset >> s->cluster_bits,
+                            l2_refcount, QCOW2_DISCARD_OTHER);
+                    if (ret < 0) {
+                        goto fail;
+                    }
                 }
+
+                offset = zeroed_cluster_offset;
+                last_zeroed_cluster_l1i = i;
+                last_zeroed_cluster_l2i = j;
             }
 
-            ret = qcow2_pre_write_overlap_check(bs, 0, offset, 
s->cluster_size);
-            if (ret < 0) {
-                if (!preallocated) {
-                    qcow2_free_clusters(bs, offset, s->cluster_size,
-                                        QCOW2_DISCARD_ALWAYS);
+            if (preallocated) {
+                ret = qcow2_pre_write_overlap_check(bs, 0, offset,
+                                                    s->cluster_size);
+                if (ret < 0) {
+                    goto fail;
                 }
-                goto fail;
-            }
 
-            ret = bdrv_write_zeroes(bs->file, offset / BDRV_SECTOR_SIZE,
-                                    s->cluster_sectors, 0);
-            if (ret < 0) {
-                if (!preallocated) {
-                    qcow2_free_clusters(bs, offset, s->cluster_size,
-                                        QCOW2_DISCARD_ALWAYS);
+                ret = bdrv_write_zeroes(bs->file, offset / BDRV_SECTOR_SIZE,
+                                        s->cluster_sectors, 0);
+                if (ret < 0) {
+                    goto fail;
                 }
-                goto fail;
             }
 
-            if (l2_refcount == 1) {
-                l2_table[j] = cpu_to_be64(offset | QCOW_OFLAG_COPIED);
+            if (preallocated) {
+                l2_table[j] = cpu_to_be64(offset | (l2_entry & 
QCOW_OFLAG_COPIED));
             } else {
                 l2_table[j] = cpu_to_be64(offset);
             }
@@ -1670,8 +1705,8 @@ static int expand_zero_clusters_in_l1(BlockDriverState 
*bs, uint64_t *l1_table,
                 qcow2_cache_depends_on_flush(s->l2_table_cache);
             }
             ret = qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
+            l2_table = NULL;
             if (ret < 0) {
-                l2_table = NULL;
                 goto fail;
             }
         } else {
@@ -1697,6 +1732,36 @@ static int expand_zero_clusters_in_l1(BlockDriverState 
*bs, uint64_t *l1_table,
         }
     }
 
+    /* Fix COPIED (only valid for active L2 tables) */
+    if (is_active_l1 && zeroed_cluster_refcount == 1) {
+        uint64_t l2_offset, l2_entry;
+
+        l2_offset = l1_table[last_zeroed_cluster_l1i] & L1E_OFFSET_MASK;
+        assert(l2_offset);
+
+        ret = qcow2_cache_get(bs, s->l2_table_cache, l2_offset,
+                              (void **)&l2_table);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        l2_entry = be64_to_cpu(l2_table[last_zeroed_cluster_l2i]);
+
+        assert(!(l2_entry & QCOW_OFLAG_COPIED));
+        l2_entry |= QCOW_OFLAG_COPIED;
+
+        l2_table[last_zeroed_cluster_l2i] = cpu_to_be64(l2_entry);
+
+        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
+        qcow2_cache_depends_on_flush(s->l2_table_cache);
+
+        ret = qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
+        l2_table = NULL;
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
     ret = 0;
 
 fail:
-- 
2.0.1