Re: [RFC PATCH 08/22] nbd: Add max-connections to nbd-server-start

[Eric Blake <eblake@redhat.com>]

On 8/13/20 11:29 AM, Kevin Wolf wrote:
> This is a QMP equivalent of qemu-nbd's --share option, limiting the
> maximum number of clients that can attach at the same time.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   qapi/block-export.json         | 10 ++++++++--
>   include/block/nbd.h            |  3 ++-
>   block/monitor/block-hmp-cmds.c |  2 +-
>   blockdev-nbd.c                 | 33 ++++++++++++++++++++++++++-------
>   qemu-storage-daemon.c          |  4 ++--
>   5 files changed, 39 insertions(+), 13 deletions(-)
> 
> diff --git a/qapi/block-export.json b/qapi/block-export.json
> index 40369814b4..1fdc55c53a 100644
> --- a/qapi/block-export.json
> +++ b/qapi/block-export.json
> @@ -14,6 +14,8 @@
>   #             is only resolved at time of use, so can be deleted and
>   #             recreated on the fly while the NBD server is active.
>   #             If missing, it will default to denying access (since 4.0).
> +# @max-connections: The maximum number of connections to allow at the same
> +#                   time, 0 for unlimited. (since 5.2; default: 0)

Nice way to add feature parity.

Limiting the number of connections (particularly for a writable client, 
where we cannot guarantee cache consistency between the connections), 
seems like a worthwhile feature to have; I've always found it odd that 
qemu-nbd and QMP nbd-server-add defaulted to different limits (1 vs. 
unlimited).  For reference, nbdkit defaults to unlimited, and I'm happy 
if qemu-storage-daemon does likewise; but changing qemu-nbd's default of 
1 would be backwards incompatible and may cause surprises (there's 
always 'qemu-nbd -e' when needed).  I also wonder if we should change 
'qemu-nbd -e 0' to mean unlimited rather than an error (right now, 
qemu-iotests/common.rc uses -e 42 for all nbd-based tests for a saner 
limit than just 1, but it smells of being arbitrary compared to unlimited).

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org