[Qemu-devel] [PATCH 1/3] l2_allocate: Write complete sectors

[Kevin Wolf <kwolf@redhat.com>]

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: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-cluster.c |   33 ++++++++++++++++++++++++++++-----
 1 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 99215fa..1c70693 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -171,6 +171,31 @@ static uint64_t *l2_load(BlockDriverState *bs, uint64_t l2_offset)
 }
 
 /*
+ * 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
  *
  * Allocate a new l2 entry in the file. If l1_index points to an already
@@ -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 */
 
-- 
1.6.0.6