On Mon, May 08, 2006 at 11:05:00PM +0100, Michael McConnell wrote:
> IIRC creating a "raw" QEMU disc image makes use of sparse files, a concept
> not supported under FAT16/32. A qcow disc image should work fine. If you
> want to create a raw disc image on a FAT partition, use (from your example)
> dd if=/dev/zero of=/mnt/partitions/windows0/qmeu-disk bs=1024 count=40960
>
> It'll take a bit longer than qemu-img would but then it's having to write out
> every block in the disc image to the real disc.
>
> Hope that helps.
>
Here is a patch that fixes the raw block driver. It is able to detect when
creation of a sparse file failed and failback to using the dd method.
qemu-img works correctly with this patch.
--
Infinite complexity begets infinite beauty.
Infinite precision begets infinite perfection.
--- block.c.orig Mon May 8 18:34:02 2006
+++ block.c Mon May 8 18:44:18 2006
@@ -756,7 +756,8 @@
static int raw_create(const char *filename, int64_t total_size,
const char *backing_file, int flags)
{
- int fd;
+ int fd, size, i;
+ unsigned char buf512[512];
if (flags || backing_file)
return -ENOTSUP;
@@ -767,6 +768,27 @@
return -EIO;
ftruncate(fd, total_size * 512);
close(fd);
+
+ /* check to see if the filesystem handled sparseness correctly */
+ fd = open(filename, O_RDONLY | O_BINARY | O_LARGEFILE);
+ if (fd < 0)
+ return -EIO; // some weird badness happened here
+ size = lseek(fd, 0LL, SEEK_END);
+ close(fd);
+
+ if (size)
+ return 0;
+ printf("Warning: your filesystem does not appear to support sparse
file\nFalling back to pseudo-/dev/zero method\nSit back and enjoy a cup of
coffee... This may take a while.\n");
+
+ fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY | O_LARGEFILE,
+ 0644);
+ if (fd < 0)
+ return -EIO;
+ memset(buf512, 0, 512);
+ for (i = 0; i < total_size; i++)
+ write(fd, buf512, 512);
+ close(fd);
+
return 0;
}
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