[sheepdog] [PATCH v2] sheepdog: do not blindly memset all read buffers

[Christoph Hellwig]

Only buffers that map to unallocated blocks need to be zeroed.

Signed-off-by: Christoph Hellwig <hch at lst.de>

---
 block/sheepdog.c |   28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

Index: qemu/block/sheepdog.c
===================================================================
--- qemu.orig/block/sheepdog.c	2012-07-08 21:20:12.896597656 +0200
+++ qemu/block/sheepdog.c	2012-07-08 21:20:56.563264061 +0200
@@ -1556,18 +1556,26 @@ static int coroutine_fn sd_co_rw_vector(
 
         len = MIN(total - done, SD_DATA_OBJ_SIZE - offset);
 
-        if (!inode->data_vdi_id[idx]) {
-            if (acb->aiocb_type == AIOCB_READ_UDATA) {
+        switch (acb->aiocb_type) {
+        case AIOCB_READ_UDATA:
+            if (!inode->data_vdi_id[idx]) {
+                qemu_iovec_memset_skip(acb->qiov, 0, len, done);
                 goto done;
             }
-
-            create = 1;
-        } else if (acb->aiocb_type == AIOCB_WRITE_UDATA
-                   && !is_data_obj_writable(inode, idx)) {
-            /* Copy-On-Write */
-            create = 1;
-            old_oid = oid;
-            flags = SD_FLAG_CMD_COW;
+            break;
+        case AIOCB_WRITE_UDATA:
+            if (!inode->data_vdi_id[idx]) {
+                create = 1;
+            }
+            if (!is_data_obj_writable(inode, idx)) {
+                /* Copy-On-Write */
+                create = 1;
+                old_oid = oid;
+                flags = SD_FLAG_CMD_COW;
+            }
+            break;
+        default:
+            break;
         }
 
         if (create) {