[Qemu-devel] RFC: No write zero patch with new AIO

[Brad Campbell <brad@wasp.net.au>]

This is not a real patch, it's been hand edited and badly whitespace damaged to remove some of my 
braindamaged/commented out debugging code that just muddies things up.

Simply posted for comment. I've tested it with WinXP and Ubuntu 6.06 and it appears to do what it's 
supposed to with no ill effects. I'd like someone familiar with the AIO system to comment if you 
have time.. particularly the way I'm skipping clusters and ending the AIO transaction.

It has been *heavily* tested though.

This is a very loose "adaption" of the zero-wipe patch by Frediano.Ziglio@vodafone.com

Index: block-qcow2.c
===================================================================
RCS file: /cvsroot/qemu/qemu/block-qcow2.c,v
retrieving revision 1.4
diff -u -r1.4 block-qcow2.c
--- block-qcow2.c       7 Aug 2006 02:38:06 -0000       1.4
+++ block-qcow2.c       12 Feb 2007 05:25:14 -0000
@@ -756,23 +756,58 @@
      return 0;
  }

+
+static int is_not_zero(const uint8_t *data, int len)
+{
+    int left;
+    while (len && (((unsigned int) data) & 3) != 0) {
+        if (*data++)
+            return 1;
+        --len;
+    }
+    left = len & 3;
+    len >>= 2;
+    while (len) {
+        if (*((uint32_t *)data) != 0)
+            return 1;
+        data += 4;
+        --len;
+    }
+    while (left) {
+        if (*data++)
+            return 1;
+        --left;
+    }
+    return 0;
+}
+
+

@@ -928,6 +964,7 @@
      int index_in_cluster;
      uint64_t cluster_offset;
      const uint8_t *src_buf;
+    int not_zero;

      acb->hd_aiocb = NULL;

@@ -937,29 +974,42 @@
          qemu_aio_release(acb);
          return;
      }
-
+next:
      acb->nb_sectors -= acb->n;
      acb->sector_num += acb->n;
      acb->buf += acb->n * 512;

      if (acb->nb_sectors == 0) {
          /* request completed */
          acb->common.cb(acb->common.opaque, 0);
          qemu_aio_release(acb);
          return;
      }

      index_in_cluster = acb->sector_num & (s->cluster_sectors - 1);
      acb->n = s->cluster_sectors - index_in_cluster;
      if (acb->n > acb->nb_sectors)
          acb->n = acb->nb_sectors;
-    cluster_offset = get_cluster_offset(bs, acb->sector_num << 9, 1, 0,
+
+    not_zero = is_not_zero(acb->buf, acb->n * 512);
+    cluster_offset = get_cluster_offset(bs, acb->sector_num << 9, not_zero, 0,
                                          index_in_cluster,
                                          index_in_cluster + acb->n);
+
      if (!cluster_offset || (cluster_offset & 511) != 0) {
-        ret = -EIO;
+        if (not_zero)
+               ret = -EIO;
+       else
+               if (acb->nb_sectors - acb->n == 0) {
+                       acb->common.cb(acb->common.opaque, 0);
+                       qemu_aio_release(acb);
+                       return;
+               }
+               goto next;
          goto fail;
      }
+
      if (s->crypt_method) {
          if (!acb->cluster_data) {
              acb->cluster_data = qemu_mallocz(s->cluster_size);

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