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Re: multiple connections per block device with NBD?


From: Eric Blake
Subject: Re: multiple connections per block device with NBD?
Date: Wed, 2 Feb 2022 07:50:31 -0600
User-agent: NeoMutt/20211029-256-77b59a

On Wed, Feb 02, 2022 at 02:42:02AM +0530, Sam wrote:
> Hello,
> 
> I am wondering whether NBD integration of qemu is able to establish multiple 
> connections to remote NBD server for a single block device. nbd-client has 
> this option:
> 
> -C, --connections
> Use num connections to the server, to allow speeding up request handling, at 
> the cost of higher resource usage on the server. Use of this option requires 
> kernel support available first with Linux 4.9.
> 
> I am looking for equivalent functionality in qemu's direct NBD access as 
> multiple connections in nbd-client does increase speeds many folds.

At the moment, qemu does not support multiple NBD client connections
to a single NBD server.  There is certainly room for improvement to
implement this, but it may not be trivial, and I am not aware of
anyone that has tried doing it yet.

For comparison, 'qemu-img convert' with an NBD destination is an
example of a single NBD connection to the server, while 'nbdcopy'
(from the libnbd project) opens multiple client connections; you can
indeed see performance improvements with nbdcopy taking advantage of
the parallelism made possible by multiple sockets.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org




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