Re: [Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE trimming

From: Stefan Hajnoczi <stefanha@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE trimming
Date: Fri, 9 Mar 2012 10:53:31 +0000	[thread overview]
Message-ID: <CAJSP0QWBbsd_sTotkdnjvn93wGH-OwZc=ek8sGUcw0Hki+TKww@mail.gmail.com> (raw)
In-Reply-To: <4F59DEC6.8010509@redhat.com>

On Fri, Mar 9, 2012 at 10:43 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> Il 09/03/2012 11:31, Stefan Hajnoczi ha scritto:
>>> > +#ifdef FALLOC_FL_PUNCH_HOLE
>>> > +    retval = fallocate(s->fd, FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE,
>>> > +                       sector_num << 9, (int64_t)nb_sectors << 9);
>> I'm concerned about introducing blocking syscalls in coroutine code
>> paths.  This needs to be done asynchronously.
>
> Right; it is no worse than what is already there, except that XFS could
> use paio_ioctl.  Alternatives are:

It is worse in the sense that if you make this feature more widely
available then it deserves a production-quality implementation.

> 1) require a new-enough kernel and only use fallocate; return a NULL
> aiocb if !has_discard and convert it to ENOTSUP.
>
> 2) extract now from my threads branch the work to generalize
> posix-aio-compat into a more flexible threadpool, and move the AIO code
> back to block/raw-posix.c.

3) Add discard to posix-aio-compat.c as an AIO request?

Stefan