Re: [Qemu-devel] Re: qemu disk on vfat

["Jim C. Brown" <jma5@umd.edu>]

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On Mon, May 08, 2006 at 08:36:15PM -0500, Anthony Liguori wrote:
> Jim C. Brown wrote:
> >Aactually, the bug is in vfat not in qemu-img.
> >
>
> Not really.  POSIX doesn't mandate that ftruncate() increase a file
> size.  This is a Linux-ism and is only valid for filesystems that
> support holes (which vfat doesn't).
>
> Regards,
>
> Anthony Liguori
>

Ok, so in that case this is something qemu-img should handle on its own then.
(Since we're not likely to see a "fix" in either glibc or the kernel for this,
and it has the potential to be a portability issue.)

On Mon, May 08, 2006 at 07:50:24PM -0400, Jim C. Brown wrote:
> qemu-img correctly uses ftruncate() which is suppose to make the file sparse
> if the underlying filesystem supports it, but it should fall back to adding zeros
> to the end of the file. On vfat you aren't able to seek past the end of a file
> period, so this doesn't work.

Turns out I was wrong about this too.

http://www.mail-archive.com/bug-tar@gnu.org/msg00556.html

Here is a patch that silently handles the Linux/vfat case using lseek().

-- 
Infinite complexity begets infinite beauty.
Infinite precision begets infinite perfection.

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--- block.c	Tue May 16 13:06:15 2006
+++ block.c	Tue May 16 13:07:51 2006
@@ -753,6 +753,20 @@
     close(s->fd);
 }
 
+int qemu_ftruncate(int fd, off_t length)
+/* ftruncate() isn't guarranteed to grow a file, according to POSIX. **
+** This is. */
+{
+    int res = ftruncate(fd, length);
+    if (res && (errno == EPERM))
+    {
+        if ((lseek( fd, length - 1, SEEK_SET) == (off_t)-1) ||
+       	    (write(fd, "\0", 1) == -1))
+       return -1;
+    }
+    return res;
+}
+
 static int raw_create(const char *filename, int64_t total_size,
                       const char *backing_file, int flags)
 {
@@ -765,7 +779,7 @@
               0644);
     if (fd < 0)
         return -EIO;
-    ftruncate(fd, total_size * 512);
+    qemu_ftruncate(fd, total_size * 512);
     close(fd);
     return 0;
 }