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

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

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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.

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--- 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;
 }