Re: ublk-qcow2: ublk-qcow2 is available

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Oct 04, 2022 at 01:57:50AM +0200, Denis V. Lunev wrote:
> On 10/3/22 21:53, Stefan Hajnoczi wrote:
> > On Fri, Sep 30, 2022 at 05:24:11PM +0800, Ming Lei wrote:
> > > ublk-qcow2 is available now.
> > Cool, thanks for sharing!
> yep
> 
> > > So far it provides basic read/write function, and compression and snapshot
> > > aren't supported yet. The target/backend implementation is completely
> > > based on io_uring, and share the same io_uring with ublk IO command
> > > handler, just like what ublk-loop does.
> > > 
> > > Follows the main motivations of ublk-qcow2:
> > > 
> > > - building one complicated target from scratch helps libublksrv APIs/functions
> > >    become mature/stable more quickly, since qcow2 is complicated and needs more
> > >    requirement from libublksrv compared with other simple ones(loop, null)
> > > 
> > > - there are several attempts of implementing qcow2 driver in kernel, such as
> > >    ``qloop`` [2], ``dm-qcow2`` [3] and ``in kernel qcow2(ro)`` [4], so ublk-qcow2
> > >    might useful be for covering requirement in this field
> There is one important thing to keep in mind about all partly-userspace
> implementations though:
> * any single allocation happened in the context of the
>    userspace daemon through try_to_free_pages() in
>    kernel has a possibility to trigger the operation,
>    which will require userspace daemon action, which
>    is inside the kernel now.
> * the probability of this is higher in the overcommitted
>    environment
> 
> This was the main motivation of us in favor for the in-kernel
> implementation.

CCed Josef Bacik because the Linux NBD driver has dealt with memory
reclaim hangs in the past.

Josef: Any thoughts on userspace block drivers (whether NBD or ublk) and
how to avoid hangs in memory reclaim?

Stefan

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