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Mounting partitions from a dd image

assafmo.github.io

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Re: Mounting partitions from a dd image

#4

Post by discreditable »

This is doing it the hard way. Make the image a loop device:

   losetup /dev/loop0 /path/to/your/image.img
Now you can use /dev/loop0 like it's a normal disk. If you're not seeing your partitions (/dev/loop0p1, etc.):

    partprobe /dev/loop0
When you're done, detach the loop device:

    losetup -d /dev/loop0
I use this trick to transparently compress ddrescue images with brtfs: https://brashear.me/blog/2017/11/14/how-to-create-a-compress...

Re: Mounting partitions from a dd image

#5

Post by peterwwillis »

amelius wrote: post #2

Is it also possible to repartition the image with conventional tools? How about setting a boot flag and writing it back to e.g. an USB stick?

Yes, you can use losetup.

  ~# dd if=/dev/zero of=image.img bs=1M count=100
  ~# LODEVICE=$(losetup --find --show image.img)
  ~# fdisk $LODEVICE
  ~# losetup -d $LODEVICE
^ The above creates a 100MB disk image, binds it to a loopback device, runs fdisk, and then detaches it. I'm not sure how formatting the partition would work but presumably those tools have an offset option too?

edit: this page explains using offsets for mke2fs and doesn't require losetup: https://superuser.com/questions/737072/userspace-manipulatio...

also, losetup can now scan the device for partition tables and make partition-specific loopbacks! https://stackoverflow.com/a/15200862

Re: Mounting partitions from a dd image

#6

Post by mrrsm »

discreditable wrote: post #4

This is doing it the hard way. Make the image a loop device: losetup /dev/loop0 /path/to/your/image.img Now you can use /dev/loop0 like it's a normal disk. If you're not seeing your partitions (/dev/loop0p1, etc.): partprobe /dev/loop0 When you're done, detach the loop device: losetup -d /dev/loop0 I use this trick to transparently compress ddrescue images with brtfs: https://brashear.me/blog/2017/11/14/how-to-create…

I came here to say using the loop device just makes things much easier. No math to get your offsets and fat fingering the numbers this way.

Re: Mounting partitions from a dd image

#8

Post by dbolgheroni »

OpenBSD uses vnconfig (also FreeBSD; on NetBSD it's vndconfig). An example:

  # vnconfig vnd0 install63.fs
Then you can treat vnd0 as if it was a disk:

  # disklabel vnd0
  # /dev/rvnd0c:
  type: vnd
  disk: vnd device
  label: fictitious
  duid: 138b4f2a2e184426
  flags:
  bytes/sector: 512
  sectors/track: 100
  tracks/cylinder: 1
  sectors/cylinder: 100
  cylinders: 7382
  total sectors: 738240
  boundstart: 1024
  boundend: 737280
  drivedata: 0 
  
  16 partitions:
  #                size           offset  fstype [fsize bsize   cpg]
    a:           736256             1024  4.2BSD   2048 16384 16142 
    c:           738240                0  unused                    
    i:              960               64   MSDOS
For instance, to mount the 'a' partition:

  # mount /dev/vnd0a /mnt
You can also associate an encryption key with the device. All data written to the diskimage will then be encrypted.

Re: Mounting partitions from a dd image

#10

Post by phaedrus »

One time I tried to create a tool to do in-place conversion between Linux software RAID volumes (e.g. RAID-0 to RAID-1 (assuming data fits), or RAID-1 to RAID-5, etc.) I used disk image files and the loopback device to test it to avoid having to reformat physical partitions.

Despite reading all of the documentation I could find, and even going so far as to read kernel source code, I could never get the system to recognize my striped block data. That is, I tried to generate three files composing a RAID-5 "by hand", but mdadm would refuse to mount them. I never figured out if there was some additional id-block I was missing, or if my striping algorithm was not correct.

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