Creating Disk Images Revision as of Sunday, 20 December 2015 at 19:56 UTC
Floppy/CD/DVD drives
Creating ISOs
The venerable dd is fantastic for this. Let’s say your device is
/dev/cdrom:
dd if=/dev/cdrom of=new_cdrom.iso
This will create an ISO9660-compliant image.
Mounting ISOs
ISO files can be loop
mounted like so:
mount -o loop new_cdrom.iso /mount/point
A random bunch of files/directories
For this example, I want to create an ISO9660-compliant image of a
directory, /home/nikhil/Documents:
mkisofs -o /home/nikhil/Documents_backup.iso /home/nikhil/Documents
Important: mkisofs can only create ISOs
from directory trees that are a maximum of 6 levels deep! Any more
and it will croak.
Hard Drives
This is quite tricky. You might want to consider
Mondo for its awesomeness. While it
does create ISOs, their contents are compressed and divvied up into
many separate files. If you want an ‘image’ that you can carry around
and mount, read on.
Using dd
Let’s say your HDD is labeled /dev/sdb. You can use dd to create a
block-by-block replica image of this drive like so:
dd if=/dev/sdb of=hard_drive_backup.img conv=sync,noerror
Simple, eh? What’s tricky is that dd will go through all blocks,
even the unallocated ones. This means that if you have a 300GB drive
with just 10GB of data, you’re going to get an image that’s 300GB in
size (can you see why Mondo or Ghost make more sense?)
The conv=sync,noerror will tell dd to write ‘something’ to the image
in case it encounters a bad block.
Speeding up the process
We could use a bigger block size. Using the default 512-byte block size
can slow things down considerably.
dd if=/dev/sdb of=hard_drive_backup.img conv=sync,noerror bs=64K
Making the process space-efficient
We could also use good ‘ol gzip or bzip2 like so:
dd if=/dev/sdb conv=sync,noerror bs=64K | gzip -c >
hard_drive_backup.img.gz
Mounting a dd image
The resultant image from the previous steps is absolutely not
ISO9660-compliant. Any attempts to mount it as such might result in mild
embarrassment. To use the image, try fdisk:
[user@localhost mondo_archives]# fdisk -ul hard_drive_backup.img
last_lba(): I don't know how to handle files with mode 81a4
You must set cylinders.
You can do this from the extra functions menu.
Disk hard_drive_backup.img: 0 MB, 0 bytes
255 heads, 63 sectors/track, 0 cylinders, total 0 sectors
Units = sectors of 1 * 512 = 512 bytes
Device Boot Start End Blocks Id System
hard_drive_backup.disk1 * 63 208844 104391 83 Linux
hard_drive_backup.disk2 208845 2313359 1052257+ 82 Linux swap / Solaris
hard_drive_backup.disk3 2313360 78156224 37921432+ 83
Linux
Partition 3 has different physical/logical endings:
phys=(1023, 254, 63) logical=(4864, 254, 63)
The tricky part
To mount the third partition (you know, the one with all the data on
it), you need some quick arithmetic: Multiply the starting cylinder by
the block size and use it as the offset for the mount command.
For example: In the output above, the offset would be
512 x 2313360 = 1184440320. You would then use this value like so:
mount -o loop,offset=1184440320 hard_drive_backup.img /mount/point