qcow2: Support exact L1 table growth

The L1 table grow operation includes a size calculation that bumps up
the new L1 table size in order to anticipate the size needs of vmstate
data.  This helps reduce the number of times that the L1 table has to be
grown when vmstate data is appended.

This size overhead is not necessary during image creation,
bdrv_truncate(), or snapshot goto operations.  In fact, existing
qemu-iotests that exercise table growth are no longer able to trigger it
because image creation preallocates an L1 table that is too large after
changes to qcow_create2().

This patch keeps the size calculation but also adds exact growth for
callers that do not want to inflate the L1 table size unnecessarily.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index fb4224a669ebeeff8e8b6545008aaf93fd6e2f0f..4f7dc59b7636f63e2ee5565e581e51af541122c9 100644 (file)
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -28,7 +28,7 @@
 #include "block_int.h"
 #include "block/qcow2.h"
 
-int qcow2_grow_l1_table(BlockDriverState *bs, int min_size)
+int qcow2_grow_l1_table(BlockDriverState *bs, int min_size, bool exact_size)
 {
     BDRVQcowState *s = bs->opaque;
     int new_l1_size, new_l1_size2, ret, i;
@@ -36,15 +36,22 @@ int qcow2_grow_l1_table(BlockDriverState *bs, int min_size)
     int64_t new_l1_table_offset;
     uint8_t data[12];
 
-    new_l1_size = s->l1_size;
-    if (min_size <= new_l1_size)
+    if (min_size <= s->l1_size)
         return 0;
-    if (new_l1_size == 0) {
-        new_l1_size = 1;
-    }
-    while (min_size > new_l1_size) {
-        new_l1_size = (new_l1_size * 3 + 1) / 2;
+
+    if (exact_size) {
+        new_l1_size = min_size;
+    } else {
+        /* Bump size up to reduce the number of times we have to grow */
+        new_l1_size = s->l1_size;
+        if (new_l1_size == 0) {
+            new_l1_size = 1;
+        }
+        while (min_size > new_l1_size) {
+            new_l1_size = (new_l1_size * 3 + 1) / 2;
+        }
     }
+
 #ifdef DEBUG_ALLOC2
     printf("grow l1_table from %d to %d\n", s->l1_size, new_l1_size);
 #endif
@@ -550,7 +557,7 @@ static int get_cluster_table(BlockDriverState *bs, uint64_t offset,
 
     l1_index = offset >> (s->l2_bits + s->cluster_bits);
     if (l1_index >= s->l1_size) {
-        ret = qcow2_grow_l1_table(bs, l1_index + 1);
+        ret = qcow2_grow_l1_table(bs, l1_index + 1, false);
         if (ret < 0) {
             return ret;
         }

diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index bbfcaaae22a0db1094d93874e05565466ecf5155..8dd5df0bcda379945050341d6dcecce458af358c 100644 (file)
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -327,7 +327,7 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
     if (qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, -1) < 0)
         goto fail;
 
-    if (qcow2_grow_l1_table(bs, sn->l1_size) < 0)
+    if (qcow2_grow_l1_table(bs, sn->l1_size, true) < 0)
         goto fail;
 
     s->l1_size = sn->l1_size;

diff --git a/block/qcow2.c b/block/qcow2.c
index ee3481b786f0e05bbf5c8c52d90d2ed61917ee22..345d2e4c3b3836ce83540bd53896b3dbf2e9b55e 100644 (file)
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1154,7 +1154,7 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
     }
 
     new_l1_size = size_to_l1(s, offset);
-    ret = qcow2_grow_l1_table(bs, new_l1_size);
+    ret = qcow2_grow_l1_table(bs, new_l1_size, true);
     if (ret < 0) {
         return ret;
     }

diff --git a/block/qcow2.h b/block/qcow2.h
index 356a34af434a73d2800a3444a00f61936240093f..add710ba7632a0f493a4832ddcaac7d487ac94a8 100644 (file)
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -188,7 +188,7 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
 int qcow2_check_refcounts(BlockDriverState *bs, BdrvCheckResult *res);
 
 /* qcow2-cluster.c functions */
-int qcow2_grow_l1_table(BlockDriverState *bs, int min_size);
+int qcow2_grow_l1_table(BlockDriverState *bs, int min_size, bool exact_size);
 void qcow2_l2_cache_reset(BlockDriverState *bs);
 int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset);
 void qcow2_encrypt_sectors(BDRVQcowState *s, int64_t sector_num,