[Xen-users] Increase size of file-based diskimage (with MBR, partitions + fs)

[Henrik Holmboe <henrik@xxxxxxxxxx>]

Hello all,

I have been struggling with a problem for some time and haven't quite
found a working solution. Tried to search the mailing list and other
online resources but still got stuck.

What I have: a diskimage stored in the fs on dom0. The image is not
just a filesystem, but contains a full disk presented to the domU. Thus
it contains an MBR, partition table and a single ext3 filesystem. No 
LVM is not used in the domU. Both dom0 and domU is Centos 5.2.

What I want to accomplish: increase the size of this diskimage, resize
the partition and filesystem.

Some basic facts and proof of valid diskimage from dom0:

 # Partition table
 
 > sfdisk -l xvda.img
 [...]
 Disk xvda.img: 195 cylinders, 255 heads, 63 sectors/track
 Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0
    Device Boot Start     End   #cyls    #blocks   Id  System
    xvda.img1   *      0+    194     195-   1566306   83  Linux
 [...]

 # Verify partition table

 > losetup /dev/loop15 xvda.img
 > sfdisk -V /dev/loop15
 /dev/loop15: OK
 > losetup -d /dev/loop15

 # Verify filesystem

 > losetup /dev/loop16 -o `expr 63 \* 512` xvda.img
 > e2fsck -f /dev/loop16
 e2fsck 1.39 (29-May-2006)
 Pass 1: Checking inodes, blocks, and sizes
 Pass 2: Checking directory structure
 Pass 3: Checking directory connectivity
 Pass 4: Checking reference counts
 Pass 5: Checking group summary information
 /: 32573/391680 files (2.8% non-contiguous), 265879/391576 blocks
 > losetup -d /dev/loop16

I can boot this image correctly using pygrub. The configuration is
simple (xvda is the fs, xvdb is the swap):

 # /etc/xen/test.cfg
 name = "test"
 memory = 256
 disk = [ 'tap:aio:/var/lib/xen/images/test/xvda.img,xvda,w',
          'tap:aio:/var/lib/xen/images/test/xvdb.img,xvdb,w', ]
 vif = [ 'mac=00:16:3E:4B:79:7B,bridge=xenbr1', ]
 bootloader = "/usr/bin/pygrub"
 vcpus = 1
 on_reboot = 'restart'
 on_crash = 'restart'

Ok, so now on to what I have tried and failed (with a known good image
as verified above):

 # Extend the size of diskimage from 1.5GB to 3GB

 > ls -la xvda.img 
 -rw-r--r-- 1 root root 1610612736 Oct  1 00:16 xvda.img
 > dd if=/dev/zero of=xvda.img bs=1 count=1 seek=3G conv=notrunc
 1+0 records in
 1+0 records out
 1 byte (1 B) copied, 0.00506 seconds, 0.2 kB/s
 > ls -la xvda.img
 -rw-r--r-- 1 root root 3221225473 Oct  1 00:25 xvda.img

 # Now rewrite the partition table to span the entire disk, also
 # make sure that the partition is still bootable

 > sfdisk -q --no-reread xvda.img << EOF
 ,,,*
 EOF
 > sfdisk -l xvda.img
 Disk xvda.img: 391 cylinders, 255 heads, 63 sectors/track
 Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0
   Device Boot Start     End   #cyls    #blocks   Id  System
   xvda.img1   *      0+    390     391-   3140707   83  Linux

 # Resize the filesystem to span the entire partition, also verify the
 # filesystem

 > losetup /dev/loop16 -o `expr 63 \* 512` xvda.img
 > e2fsck -f /dev/loop16
 [...passed]
 > resize2fs /dev/loop16
 resize2fs 1.39 (29-May-2006)
 Resizing the filesystem on /dev/loop16 to 786424 (4k) blocks.
 The filesystem on /dev/loop16 is now 786424 blocks long.
 > e2fsck -f /dev/loop16
 [...passed]
 > sfdisk -V xvda.img
 xvda.img: OK

So far so good! I can mount the filesystem and verify it to be 3GB. Now
on to trying to boot the domU again:

 > xm create -c hc24.cfg
 Using config file "./test.cfg".
 Traceback (most recent call last):
   File "/usr/bin/pygrub", line 651, in ?
       fs = fsimage.open(file, get_fs_offset(file))
       IOError: [Errno 95] Operation not supported
       No handlers could be found for logger "xend"
       Error: Boot loader didn't return any data!

If I understand this correctly the diskimage does not contain a valid
MBR that satisfies pygrub. I have tried two things to solve this:

TEST1

 # Try to copy the MBR from backup to the new diskimage

 > dd if=oldxvda.img of=xvda.img count=1 bs=446 conv=notrunc

As I understand it the first 446 bytes contain the MBR, while the first
512 bytes would also copy the old partition table. Trying to boot this
disk image gives the same error message as before.

TEST2

 # Try to create a new MBR on the disk, but for whatever reason GRUB
 # cannot recognize the filesystem, even though it finds the partition
 # type

 > losetup /dev/loop15 xvda.img
 > grub --batch --device-map=/dev/null
 [...]
 grub> device (hd0) /dev/loop15
 device (hd0) /dev/loop15
 grub> root (hd0,0)
  Filesystem type unknown, partition type 0x83
 grub> setup (hd0)
  Error 17: Cannot mount selected partition
 grub> quit
 > losetup -d /dev/loop15

So this fails miserably and would of course not start either.

Folks, what am I missing?

Thanks,
Henrik

PS. Thanks for reading this whole thing... ;(

-- 
Henrik Holmboe, Stockholm, Sweden
<http://henrik.holmboe.se/contact/>

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