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Date: Tue, 22 Feb 2011 18:07:35 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
Message-ID: <20110222210735.GA9372@amt.cnet>
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	<4D642181.4080509@codemonkey.ws>
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Subject: [Qemu-devel] Re: [patch 2/3] Add support for live block copy
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Jes.Sorensen@redhat.com, qemu-devel@nongnu.org, avi@redhat.com

On Tue, Feb 22, 2011 at 02:50:09PM -0600, Anthony Liguori wrote:
> >+static int write_commit_file(BdrvCopyState *s)
> >+{
> >+    char commit_msg[1400];
> >+    const char *buf = commit_msg;
> >+    int len, ret;
> >+
> >+    sprintf(commit_msg, "commit QEMU block_copy %s ->  %s\n", s->src_device_name,
> >+                        s->dst_filename);
> >+
> >+    len = strlen(commit_msg);
> >+    while (len>  0) {
> >+        ret = write(s->commit_fd, buf, len);
> >+        if (ret == -1&&  errno == EINTR) {
> >+            continue;
> >+        }
> >+        if (ret<= 0) {
> >+            return -errno;
> >+        }
> >+        buf += ret;
> >+        len -= ret;
> >+    }
> >+
> >+    if (fsync(s->commit_fd) == -1) {
> >+        return -errno;
> >+    }
> >
> 
> This is more or less black magic.  What is this commit file used for
> and why aren't we using something like a QMP event?

The commit file is considered reliable storage for the result of image
switch operation. Think of the following scenario:

- mgmt app requests live copy of guests ide1-hd0 
from /a/image.img to /b/image.img.
- mgmt app dies.
- guest switches to /b/image.img, /a/image.img is outdated.
- guest dies.

Notifying the switch via QMP would not be reliable in this case.

Will add this as a comment in the code.