UMdev

From VirtualSquare

Umdev creates virtual devices.

$ um_add_service umdev

[edit] Virtually partitioning a Disk Image

First of all an ampty file can be created with this command:

$ dd of=/tmp/diskimage bs=1024 count=0 seek=1024000

This file appears as a 1G sequence of null Bytes (in the reality it uses very few data blocks, it is a sparse file).

It is possible to see the file as a disk using the umdevmbr module.

$ mount -t umdevmbr /tmp/diskimage /dev/hda

The virtual disk /dev/hda can be partitioned in the usual way:

$ /sbin/fdisk /dev/hda
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.

Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-127, default 1): 
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-127, default 127): 100

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
e
Partition number (1-4): 2
First cylinder (101-127, default 101): 
Using default value 101
Last cylinder or +size or +sizeM or +sizeK (101-127, default 127): 
Using default value 127

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.
$ /sbin/fdisk /dev/hda

Command (m for help): p

Disk /dev/hda: 1048 MB, 1048576000 bytes
255 heads, 63 sectors/track, 127 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/hda1               1         100      803218+  83  Linux
/dev/hda2             101         127      216877+   5  Extended

Command (m for help): q
$

Now /dev/hda1 and /dev/hda2 are correctly defined as it were a real disk. For example it is possible to create an ext2 file system on one of the partitions:

$ /sbin/mkfs.ext2 /dev/hda1
mke2fs 1.40-WIP (07-Apr-2007)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
96384 inodes, 192612 blocks
9630 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=201326592
6 block groups
32768 blocks per group, 32768 fragments per group
16064 inodes per group
Superblock backups stored on blocks: 
        32768, 98304, 163840

Writing inode tables: done                            
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 35 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.
$

Umview modules support virtualization nesting: virtual entities provided by a module can be used to provide further virtual service. For example the virtual partition created by umbinfmt can now be mounted using umfuse.

$ um_add_service umfuse
$ mount -t umfuseext2 -o rw+ /dev/hda1 /mnt
$ ls /mnt
lost+found
$

It is possible to use further levels of nesting: if the virtual file system contains a disk image it can be mounted by umdevmbr and then one or more aprtition mounted by umfuse.