l2_allocate: Write complete sectors

When modifying the L1 table, l2_allocate() needs to write complete sectors
instead of single entries. The L1 table is already in memory, reading it from
disk in the block layer to align the request is wasted performance.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

block/qcow2-cluster.c

@@ -170,6 +170,31 @@ static uint64_t *l2_load(BlockDriverState *bs, uint64_t l2_offset)
     return l2_table;
 }
 
+/*
+ * Writes one sector of the L1 table to the disk (can't update single entries
+ * and we really don't want bdrv_pread to perform a read-modify-write)
+ */
+#define L1_ENTRIES_PER_SECTOR (512 / 8)
+static int write_l1_entry(BDRVQcowState *s, int l1_index)
+{
+    uint64_t buf[L1_ENTRIES_PER_SECTOR];
+    int l1_start_index;
+    int i;
+
+    l1_start_index = l1_index & ~(L1_ENTRIES_PER_SECTOR - 1);
+    for (i = 0; i < L1_ENTRIES_PER_SECTOR; i++) {
+        buf[i] = cpu_to_be64(s->l1_table[l1_start_index + i]);
+    }
+
+    if (bdrv_pwrite(s->hd, s->l1_table_offset + 8 * l1_start_index,
+        buf, sizeof(buf)) != sizeof(buf))
+    {
+        return -1;
+    }
+
+    return 0;
+}
+
 /*
  * l2_allocate
  *
@@ -184,7 +209,7 @@ static uint64_t *l2_allocate(BlockDriverState *bs, int l1_index)
 {
     BDRVQcowState *s = bs->opaque;
     int min_index;
-    uint64_t old_l2_offset, tmp;
+    uint64_t old_l2_offset;
     uint64_t *l2_table, l2_offset;
 
     old_l2_offset = s->l1_table[l1_index];
@@ -196,11 +221,9 @@ static uint64_t *l2_allocate(BlockDriverState *bs, int l1_index)
     /* update the L1 entry */
 
     s->l1_table[l1_index] = l2_offset | QCOW_OFLAG_COPIED;
-
-    tmp = cpu_to_be64(l2_offset | QCOW_OFLAG_COPIED);
-    if (bdrv_pwrite(s->hd, s->l1_table_offset + l1_index * sizeof(tmp),
-                    &tmp, sizeof(tmp)) != sizeof(tmp))
+    if (write_l1_entry(s, l1_index) < 0) {
         return NULL;
+    }
 
     /* allocate a new entry in the l2 cache */