[Qemu-devel] [PATCH 4/4] qcow2: detect if no disk cache

[Laurent Vivier]

pièce jointe document texte brut
(0004-qcow2-detect-if-no-disk-cache.patch)
To reduce the number of bytes read and written, disable
the forced I/O alignment when the file is not open
with O_DIRECT.

Signed-off-by: Laurent Vivier <address@hidden>
---
 block-qcow2.c |   24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

Index: qemu/block-qcow2.c
===================================================================
--- qemu.orig/block-qcow2.c     2008-11-06 16:41:15.000000000 +0100
+++ qemu/block-qcow2.c  2008-11-06 16:41:16.000000000 +0100
@@ -147,6 +147,7 @@ typedef struct BDRVQcowState {
     int snapshots_size;
     int nb_snapshots;
     QCowSnapshot *snapshots;
+    int disk_nocache;
 } BDRVQcowState;
 
 static int decompress_cluster(BDRVQcowState *s, uint64_t cluster_offset);
@@ -193,6 +194,7 @@ static int qcow_open(BlockDriverState *b
     ret = bdrv_file_open(&s->hd, filename, flags);
     if (ret < 0)
         return ret;
+    s->disk_nocache = (flags & BDRV_O_NOCACHE) != 0;
     if (bdrv_pread(s->hd, 0, &header, sizeof(header)) != sizeof(header))
         goto fail;
     be32_to_cpus(&header.magic);
@@ -994,8 +996,13 @@ static uint64_t alloc_cluster_offset(Blo
      * and s->l2_bits is (s->cluster_bits - 3) = 12 - 3 = 9;
      * and s->l2_size = 1 << s->l2_bits, so l2_cache is aligned on 512 
boundary.
      */
-    aligned_index = l2_index & ~63;
-    aligned_size = (l2_index - aligned_index + nb_clusters + 63) & ~63ULL;
+    if (s->disk_nocache) {
+        aligned_index = l2_index & ~63;
+        aligned_size = (l2_index - aligned_index + nb_clusters + 63) & ~63ULL;
+    } else {
+        aligned_index = l2_index;
+        aligned_size = nb_clusters;
+    }
     aligned_size *= sizeof(uint64_t);
     if (bdrv_pwrite(s->hd,
                     l2_offset + aligned_index * sizeof(uint64_t),
@@ -2467,10 +2474,15 @@ static int update_cluster_refcount(Block
          * size of refcount_block_cache is s->cluster_size (4096)
          * so we can align access on a 512 boundary
          */
-        aligned_index = block_index & ~((512 >> REFCOUNT_SHIFT) - 1);
-        aligned_size = (block_index - aligned_index + nb_block_index + 
-                        ((512 >> REFCOUNT_SHIFT) - 1)) &
-                        ~((512 >> REFCOUNT_SHIFT) - 1);
+        if (s->disk_nocache) {
+            aligned_index = block_index & ~((512 >> REFCOUNT_SHIFT) - 1);
+            aligned_size = (block_index - aligned_index + nb_block_index + 
+                            ((512 >> REFCOUNT_SHIFT) - 1)) &
+                            ~((512 >> REFCOUNT_SHIFT) - 1);
+        } else {
+            aligned_index = block_index;
+            aligned_size = nb_block_index;
+        }
         aligned_size *= sizeof(uint16_t);
         if (bdrv_pwrite(s->hd,
                         refcount_block_offset +

--