Tag Archives: qemu-storage-daemon

nbdkit + libblkio

Our plugin-based Network Block Device server, nbdkit, now has support for libblkio.

libblkio is a library written by Stefan Hajnoczi, Alberto Faria, Stefano Garzarella and others for accessing some somewhat unusual disk protocols including vhost-user, NVMe, vDPA, VFIO and io_uring which I’ll talk about below. It’s important to know that these are not disk formats (like raw or qcow2), but accelerated protocols for talking to virtual or real hardware.

The library is written in Rust (but offers a C API) and I believe it’s intended to replace various bottom-end parts of the qemu block layer at some point in the future.

The library uses a set of property strings to describe how to connect to a device. The nbdkit plugin maps those almost exactly into command line parameters, so you can usually follow the libblkio docs and translate that into an nbdkit command line, eg:

$ nbdkit blkio io_uring path=fedora.img

This sets the libblkio driver to “io_uring” and the path to the path of a local file. This libblkio driver uses Linux’s relatively new io_uring facility to access a local file or block device, the simplest way to use libblkio.

The other most frequently used protocol or libblkio driver is vhost-user. This is a protocol that allows a server to share a disk image to client(s) on the same machine. It uses a Unix domain socket for communication, but unlike Network Block Device (NBD) it’s not possible to use this over the network. For greater performance vhost uses shared memory between the client and server for data transfer.

qemu-storage-daemon is the most common server:

$ qemu-storage-daemon \

--blockdev driver=file,node-name=file,filename=fedora.qcow2 \

--blockdev driver=qcow2,node-name=qcow2,file=file \

--export type=vhost-user-blk,id=export,addr.type=unix,addr.path=sock,node-name=qcow2

To connect from nbdkit, just use the socket:

$ nbdkit blkio virtio-blk-vhost-user path=sock

You might wonder why we want to add libblkio support to nbdkit (apart from it being fun). There’s a practical reason which is this brings along all of the scripting support we’ve created around NBD to these somewhat obscure (albeit quite widely used) protocols. I don’t think it was possible before to use Python to script against, eg., vhost-user, but now it is:

$ nbdsh -u nbd://localhost -c 'print("%r" % h.pread(512,0))'

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