2021 update: nbdcopy is a better way to stream to and from stdin/stdout and qemu images.
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)
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