linux-kernel - Re: ublk-nbd: ublk-nbd is avaialbe

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Date:   Thu, 26 Jan 2023 05:08:22 +0100
From:   Willy Tarreau <w@....eu>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        nbd@...er.debian.org
Subject: Re: ublk-nbd: ublk-nbd is avaialbe

Hi,

On Thu, Jan 26, 2023 at 11:08:26AM +0800, Ming Lei wrote:
> Hi Jens,
> 
> On Thu, Jan 19, 2023 at 11:49:04AM -0700, Jens Axboe wrote:
> > On 1/19/23 7:23 AM, Ming Lei wrote:
> > > Hi,
> > > 
> > > ublk-nbd[1] is available now.
> > > 
> > > Basically it is one nbd client, but totally implemented in userspace,
> > > and wrt. current nbd-client in [2], the transmission phase is done
> > > by linux block nbd driver.
> > > 
> > > The handshake implementation is borrowed from nbd project[2], so
> > > basically ublk-nbd just adds new code for implementing transmission
> > > phase, and it can be thought as moving linux block nbd driver into
> > > userspace.
> > > 
> > > The added new code is basically in nbd/tgt_nbd.cpp, and io handling
> > > is based on liburing[3], and implemented by c++20 coroutine, so
> > > everything is done in single pthread totally lockless, meantime turns
> > > out it is pretty easy to design & implement, attributed to ublk framework,
> > > c++20 coroutine and liburing.
> > > 
> > > ublk-nbd supports both tcp and unix socket, and allows to enable io_uring
> > > send zero copy via command line '--send_zc', see details in README[4].
> > > 
> > > No regression is found in xfstests by using ublk-nbd as both test device
> > > and scratch device, and builtin test(make test T=nbd) runs well.
> > > 
> > > Fio test("make test T=nbd") shows that ublk-nbd performance is
> > > basically same with nbd-client/nbd driver when running fio on real
> > > ethernet link(1g, 10+g), but ublk-nbd IOPS is higher by ~40% than
> > > nbd-client(nbd driver) with 512K BS, which is because linux nbd
> > > driver sets max_sectors_kb as 64KB at default.
> > > 
> > > But when running fio over local tcp socket, it is observed in my test
> > > machine that ublk-nbd performs better than nbd-client/nbd driver,
> > > especially with 2 queue/2 jobs, and the gap could be 10% ~ 30%
> > > according to different block size.
> > 
> > This is pretty nice! Just curious, have you tried setting up your
> > ring with
> > 
> > p.flags |= IORING_SETUP_SINGLE_ISSUER | IORING_SETUP_DEFER_TASKRUN;
> > 
> > and see if that yields any extra performance improvements for you?
> > Depending on how you do processing, you should not need to do any
> > further changes there.
> > 
> > A "lighter" version is just setting IORING_SETUP_COOP_TASKRUN.
> 
> IORING_SETUP_COOP_TASKRUN is enabled in current ublksrv.
> 
> After disabling COOP_TASKRUN and enabling SINGLE_ISSUER & DEFER_TASKRUN,
> not see obvious improvement, meantime regression is observed on 64k
> rw.

Does it handle network errors better than the default nbd client, i.e.
is it able to seamlessly reconnect after while keeping the same device
or do you end up with multiple devices ? That's one big trouble I faced
with the original nbd client, forcing you to unmount and remount
everything after a network outage for example.

Thanks!
Willy