Tag Archives: streaming

Streaming NBD server

The command:

qemu-img convert input output

does not work if the output is a pipe.

It’d sure be nice if it did though! For one thing, we could use this in virt-v2v to stream images into OpenStack Glance (instead of having to spool them into a temporary file).

I mentioned this to Paolo Bonzini yesterday and he suggested a simple workaround. Just replace the output with:

qemu-img convert -n input nbd:...

and write an NBD server that turns the sequence of writes from qemu-img into a stream that gets written to a pipe. Assuming the output is raw, then qemu-img convert will write, starting at disk offset 0, linearly through to the end of the disk image.

How to write such an NBD server easily? nbdkit is a project I started to make it easy to write NBD servers.

So I wrote a streaming plugin which does exactly that, in 243 lines of code.

Using a feature called captive nbdkit, you can rewrite the above command as:

nbdkit -U - streaming pipe=/tmp/output --run '
  qemu-img convert -n input -O raw $nbd
'

(This command will “hang” when you run it — you have to attach some process to read from the pipe, eg: md5sum < /tmp/output)

Further work

The streaming plugin will a lot more generally useful if it supported a sliding window, allowing limited reverse seeking and reading. So there’s a nice little project for a motivated person. See here

5 Comments

Filed under Uncategorized