Virtual block device

block-device

I'm not sure what I should be googling or if FUSE does this (I suspect not). I'd like to create a virtual block device for which all forms of access, for example reads and writes, go directly to my app.

I know I can create a file be used as a block device by doing

dd if=/dev/zero of=~/test count=100k

then create a loopback to it using

losetup /dev/loop0 ~/test`

But I would like accesses going directly to my app instead of to a file. I hope this question is fairly clear.

Best Answer

You can use NBD. Using nbdkit you can even write virtual block devices in shell script or other scripting languages (although stick to C if you want the best performance). I gave a talk about this topic at FOSDEM 2019 where I did a live demo writing a Linux kernel block device in shell script.

Related Solutions

GNU/Linux – Overlay Block Device and Stackable Block Device

You can do that with the device mapper and its snapshot target.

Basically, you'd do the same as what LVM does when you create a writable snapshot.

dev=/dev/read-only-device
ovl=/path/to/overlay.file
newdevname=newdevice
size=$(blockdev --getsz "$dev")

loop=$(losetup -f --show -- "$ovl")
printf '%s\n' "0 $size snapshot $dev $loop P 8" |
  dmsetup create "$newdevname"

Then you can access the overlayed device as /dev/mapper/newdevice.

If you also need access to the original device at the same time, you can do:

printf '%s\n' "0 $size snapshot-origin $dev" |
  dmsetup create originaldevice

And access it over /dev/mapper/originaldevice.

You can write to that device, then in addition to the chunks written to the snapshot device, the overlay file will contain a copy of the chunks that have been overwritten when writing to the snapshot-origin.

The overlay file can be a sparse file. (for instance, create it as truncate -s10G the-file), and doesn't have to be as large as the original device. You can tell how full it is with dmsetup status "$newdevname".

Note: There are size and contents reqirements on a snapshot device.

Linux – Create a write-cache loop device for a much larger block device

You can use either dm-snapshot or NBD in copy-on-write mode.

The dm-snapshot solution is provided here (sorry for not repeating it):

https://raid.wiki.kernel.org/index.php/Recovering_a_failed_software_RAID#Making_the_harddisks_read-only_using_an_overlay_file

As for NBD, you can install nbd-server and nbd-client, and then use it like this:

mount /mnt/storage # something with some free space
losetup --read-only /dev/sda1 /dev/loop0 # to ensure it's readonly
ln -s /dev/loop0 /mnt/storage/loop0
nbd-server 127.0.0.1@4242 /mnt/storage/loop0 -c

The symlink is necessary because nbd-server insists storing the temporary write cache file to the same location as the file it is serving. So without the link it would end up in /dev/ which is not useful at all.

Finally connect to it with the client:

nbd-client 127.0.0.1 4242 /dev/nbd0

The only problem with this NBD solution is that it uses quite a lot of RAM (depending on your device size), regardless of temporary storage being available. Since fsck itself is also quite RAM hungry at times, it's possible to run out if you don't have a lot of RAM installed.

Best Answer

Related Solutions

GNU/Linux – Overlay Block Device and Stackable Block Device

Linux – Create a write-cache loop device for a much larger block device

Related Question