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Re: [PATCH v3 1/2] qemu-nbd: Use SOMAXCONN for socket listen() backlog

From: Daniel P . Berrangé
Subject: Re: [PATCH v3 1/2] qemu-nbd: Use SOMAXCONN for socket listen() backlog
Date: Tue, 9 Feb 2021 16:13:43 +0000
User-agent: Mutt/1.14.6 (2020-07-11)

On Tue, Feb 09, 2021 at 09:27:58AM -0600, Eric Blake wrote:
> Our default of a backlog of 1 connection is rather puny; it gets in
> the way when we are explicitly allowing multiple clients (such as
> qemu-nbd -e N [--shared], or nbd-server-start with its default
> "max-connections":0 for unlimited), but is even a problem when we
> stick to qemu-nbd's default of only 1 active client but use -t
> [--persistent] where a second client can start using the server once
> the first finishes.  While the effects are less noticeable on TCP
> sockets (since the client can poll() to learn when the server is ready
> again), it is definitely observable on Unix sockets, where on Unix, a
> client will fail with EAGAIN and no recourse but to sleep an arbitrary
> amount of time before retrying if the server backlog is already full.
> Since QMP nbd-server-start is always persistent, it now always
> requests a backlog of SOMAXCONN; meanwhile, qemu-nbd will request
> SOMAXCONN if persistent, otherwise its backlog should be based on the
> expected number of clients.
> See https://bugzilla.redhat.com/1925045 for a demonstration of where
> our low backlog prevents libnbd from connecting as many parallel
> clients as it wants.
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> CC: qemu-stable@nongnu.org
> ---
>  blockdev-nbd.c |  7 ++++++-
>  qemu-nbd.c     | 10 +++++++++-
>  2 files changed, 15 insertions(+), 2 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

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