Creating a modifiable gzipped disk image

Dusty Mabe set me a challenge yesterday. He wants to create several compressed disk images that have slightly different content, but are otherwise mostly the same. The disk images are large and compressing them takes a long time (30 minutes each, apparently), so ideally what we’d want to do is compress the disk image just the once and then do the updates on the gzipped image.

Modifying a file which has already been compressed is not usually possible.

However if we make some relatively uncontroversial assumptions and accept a few limitations then we can create a compressed disk image which is modifiable in this way, certainly for gzip and xz (I need to investigate zstd).

Firstly let’s assume we’re using some kind of block-based compression with fixed size blocks. This is true for gzip and xz already. Secondly let’s assume that we want to only modify a single, small partition of the image (Dusty only wants to modify the /boot partition). Lastly we’ll assume that the partition boundaries are aligned to the compression blocks. Since partitions can be placed under control of whoever creates the disk image this last one is pretty easy to arrange.

The trick is to use uncompressed blocks for the part of the input covering the partition you want to modify, and compress the rest of the file normally. Both gzip and xz have an uncompressed block type. (In fact, just about any reasonable compression algorithm works like this – if the input data doesn’t become smaller after trying to compress it, the algorithm will emit the data uncompressed since that takes less space.)

Normal tools won’t let you create files like this, but I wrote one for gzip here, and I’m confident that one could be written for xz (exercise for readers! … or me if we decide to productise this).

I created a normal Fedora 36 image using virt-builder and used guestfish to find the partition boundaries:

$ guestfish --ro -a /var/tmp/fedora-36.img -i part-list /dev/sda
[0] = {
  part_num: 1
  part_start: 1048576
  part_end: 2097151
  part_size: 1048576
}
[1] = {
  part_num: 2
  part_start: 2097152
  part_end: 1075838975
  part_size: 1073741824
}
[2] = {
  part_num: 3
  part_start: 1075838976
  part_end: 6441402367
  part_size: 5365563392
}

We will compress partition #3 while leaving partitions #1 and #2 uncompressed so they can be modified in place later. The tool is a bit crude, but:

$ ./partial-deflate /var/tmp/fedora-36.img /var/tmp/fedora-36.img.gz 0 1075838976 6441402368 

The output is a regular gzip file (albeit rather large because the first 1GB is uncompressed – if I was doing this for real I’d make that boot partition smaller):

$ ll /var/tmp/fedora-36.img*
-rw-r--r--. 1 rjones rjones 6442450944 Nov 30 17:03 /var/tmp/fedora-36.img
-rw-r--r--. 1 rjones rjones 1698849910 Dec  1 16:23 /var/tmp/fedora-36.img.gz
$ zcat /var/tmp/fedora-36.img.gz | md5sum 
57cbd5ebcbe59613378c8aee7ad9e40d  -
$ md5sum /var/tmp/fedora-36.img
57cbd5ebcbe59613378c8aee7ad9e40d  /var/tmp/fedora-36.img

Then I went ahead and modified some known content inside the gzip file (but not compressed). I used a hex editor for this, but you could play around with guestfish + nbdkit-offset-filter for something more supportable:

And the result is a gzipped file with the modifications:

$ zcat /var/tmp/fedora-36.img.gz > /var/tmp/fedora-36.img-modified
gzip: /var/tmp/fedora-36.img.gz: invalid compressed data--crc error
$ guestfish --ro -a /var/tmp/fedora-36.img-modified -i cat /boot/mydata.txt 
helloworld------

…and a CRC error. That’s to be expected, as I haven’t yet worked out how to update CRC-32 after making changes, but it should be easy to solve (with brute force if necessary).