I find myself working with Raspbian disk images fairly often. A typical workflow is:

  • Download the disk image.
  • Mount the filesystem somewhere to check something.
  • Make some changes or install packages just to check something else.
  • Crap I've made changes.

...at which point I need to fetch a new copy of the image next time I want to start fresh.

Sure, I could just make a copy of the image and work from there, but what fun is that? This seemed like a perfect opportunity to learn more about the device mapper and in particular how the snapshot target works.

Making sure we have a block device

The device mapper only operates on block devices, so the first thing we need to do is to make the source image available as a block device. We can do that with the losetup command, which maps a file to a virtual block device (or loop device).

I run something like this:

losetup --find --show --read-only 2017-11-29-raspbian-stretch-lite.img

This will find the first available block device, and then use it make my disk image available in read-only mode. Those of you who are familiar with losetup may be thinking, "you know, losetup knows how to handle partitioned devices", but I am ignoring that for the purpose of using device mapper to solve things.

Mapping a partition

We've just mapped the entire disk image to a block device. We can't use this directly because the image has multiple partitions:

# sfdisk -l /dev/loop0
Disk /dev/loop0: 1.7 GiB, 1858076672 bytes, 3629056 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x37665771

Device       Boot Start     End Sectors  Size Id Type
/dev/loop0p1       8192   93236   85045 41.5M  c W95 FAT32 (LBA)
/dev/loop0p2      94208 3629055 3534848  1.7G 83 Linux

We want to expose partition 2, which contains the root filesystem. We can see from the above output that partition 2 starts at sector 94208 and extends for 3534848 sectors (where a sector is for our purposes 512 bytes). If you need to get at this information programatically, sfdisk has --json option that can be useful; for example:

# p_start=$(sfdisk --json /dev/loop0 |
  jq ".partitiontable.partitions[1].start")
# echo $p_start

We want to expose this partition as a distinct block device. We're going to do this by creating a device mapper linear target. To create device mapper devices, we use the dmsetup command; the basic syntax is:

dmsetup create <name>

By default, this expects to read a table describing the device on stdin, although it is also possible to specify one on the command line. A table consists of one or more lines of the format:

<base> <length> <target> [<options>]

Where <base> is the starting offset in sectors for this particular segment, <length> is the length, <target> is the target type (linear, snapshot, zero, etc), and the option are specific to the particular target in use.

To create a device exposing partition 2 of our image, we run:

# dmsetup create base <<EOF
0 3534848 linear /dev/loop0 94208

This creates a device named /dev/mapper/base. Sectors 0 through 3534848 of this device will be provided by /dev/loop0, starting at offset 94208. At this point, we can actually mount the filesystem:

# mount -o ro /dev/mapper/base /mnt
# ls /mnt
bin   dev  home  lost+found  mnt  proc  run   srv  tmp  var
boot  etc  lib   media       opt  root  sbin  sys  usr
# umount /mnt

But wait, there's a problem! These disk images usually have very little free space. We're going to want to extend the length of our base device by some amount so that we have room for new packages and so forth. Fortunately, since our goal is that all writes are going to a snapshot, we don't need to use real space. We can add another segment to our table that uses the zero target.

Let's first get rid of the device we just created:

# dmsetup remove base

And create a new one:

# dmsetup create base <<EOF
0 3534848 linear /dev/loop0 94208
3534848 6950912 zero

This extends our base device out to 5G (or a total of 10485760 sectors), although attempting to read from anything beyond sector 3534848 will return zeros, and writes will be discarded. But that's okay, because the space available for writes is going to come from a COW ("copy-on-write") device associated with our snapshot: in other words, the capacity of our snapshot will be linked to size of our COW device, rather than the size of the underlying base image.

Creating a snapshot

Now that we've sorted out our base image it's time to create the snapshot device. According to the documentation, the table entry for a snapshot looks like:

snapshot <origin> <COW device> <persistent?> <chunksize>

We have our <origin> (that's the base image we created in the previous step), but what are we going to use as our <COW device>? This is a chunk of storage that will receive any writes to the snapshot device. This could be any block device (another loop device, an LVM volume, a spare disk partition), but for my purposes it seemed convenient to use a RAM disk, since I had no need for persistent changes. We can use the zram kernel module for that. Let's start by loading the module:

# modprobe zram

Without any additional parameters this will create a single RAM disk, /dev/zram0. Initially, it's not very big:

# blockdev --getsz /dev/zram0

But we can change that using the sysfs interface provided in /sys/block/zram0/. The disksize option controls the size of the disk. Let's say we want to handle up to 512M of writes; that means we need to write the value 512M to /sys/block/zram0/disksize:

echo 512M > /sys/block/zram0/disksize

And now:

# blockdev --getsz /dev/zram0

We now have all the requirements to create our snapshot device:

dmsetup create snap <<EOF
0 10485760 snapshot /dev/mapper/base /dev/zram0 N 16

This creates a device named /dev/mapper/snap. It is a 5G block device backed by /dev/mapper/base, with changes written to /dev/zram0. We can mount it:

# mount /dev/mapper/snap /mnt
# df -h /mnt
Filesystem        Size  Used Avail Use% Mounted on
/dev/mapper/snap  1.7G  943M  623M  61% /mnt

And we can resize it:

# resize2fs !$
resize2fs /dev/mapper/snap
resize2fs 1.43.3 (04-Sep-2016)
Filesystem at /dev/mapper/snap is mounted on /mnt; on-line resizing required
old_desc_blocks = 1, new_desc_blocks = 1
The filesystem on /dev/mapper/snap is now 1310720 (4k) blocks long.

# df -h /mnt
Filesystem        Size  Used Avail Use% Mounted on
/dev/mapper/snap  4.9G  944M  3.8G  20% /mnt

You'll note here that it looks like we have a 5G device, because that's the size of our base image. Because we've only allocated 512M to our COW device, we can actually only handle up to 512M of writes before we invalidate the snapshot.

We can inspect the amount of our COW device that has been consumed by changes by using dmsetup status:

# dmsetup status snap
0 10485760 snapshot 107392/1048576 0

This tells us that 107392 sectors of 1048576 total have been consumed so far (in other words, about 54M out of 512M). We can get similar information from the perspective of the zram module using zramctl:

# zramctl
/dev/zram0 lzo           512M 52.4M   34K   72K       4

This information is also available in /sys/block/zram0/mm_stat, but without any labels.

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