Requirements for an NBD client library in userspace
I believe there is no simple NBD client library. Am I wrong about
this? Let's assume I'm not for the sake of the rest of this email ...
We do however have at least 4 places this could be consumed if it existed:
- fio's proposed NBD engine
https://www.spinics.net/lists/fio/msg07831.html
- nbdkit-nbd-plugin
https://github.com/libguestfs/nbdkit/tree/master/plugins/nbd
- Martin K's forthcoming VMware liberation tool
- qemu, maybe?
https://github.com/qemu/qemu/blob/master/block/nbd-client.c
In this email I'm trying to assemble some requirements for such a
library, and I've also attempted to categorize them according to how
difficult they might be to implement. I guess everyone will have
their own opinions on which features are most "required" and which are
optional, but I've added mine below too.
Did I miss out anything?
1 = easy to implement ... 5 = hard to implement
|
| R* = required, library could not be used without it
| R = required
| O+ = optional, but very useful for many users
| O = optional
| O- = optional, not likely to be useful for many users
| |
v v
1 R* Library which runs in userspace, callable from C programs
3 O+ Callable from other programming languages, eg Python etc.
1 R* Connect to Unix domain socket
1 R* Connect to TCP port
1 R Connect to TCP port over IPv6
1 O- Connect to arbitrary file descriptor [eg. for nbdkit -s]
2 R Synchronous support for easy initial use
5 O+ Non-blocking/asynch support, use with select/poll
4 O+ Integrates with external main loops, glib2, APR pollset, etc.
4 O TLS support using X.509 certs
4 O TLS-PSK support
5 O NBD MULTI_CONN
5 O NBD out of order requests on single socket
2 O- Old-style protocol
3 R NBD_OPT_INFO mode [required for qemu]
5 O NBD structured replies
3 O+ NBD block status querying
? ? NBD resize extension
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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