Re: [PATCH v6] block/file-posix.c: Use pwritev2() with RWF_DSYNC for FUA

[Pinku Deb Nath <prantoran@gmail.com>]

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Yay! 😊

On Tue, Apr 8, 2025, 8:43 a.m. Stefan Hajnoczi <stefanha@redhat.com> wrote:

> On Mon, Apr 07, 2025 at 08:47:30AM -0700, Pinku Deb Nath wrote:
> > Full Unit Access (FUA) is an optimization where a disk write with the
> > flag set will be persisted to disk immediately instead of potentially
> > remaining in the disk's write cache.
> >
> > This commit address the todo task
> > for using pwritev2() with RWF_DSYNC in the thread pool section of
> > raw_co_prw(), if pwritev2() with RWF_DSYNC is available in the host,
> > which is always the case for Linux kernel >= 4.7.
> >
> > The intent for FUA is indicated with the BDRV_REQ_FUA flag.
> > The old code paths are preserved in case BDRV_REQ_FUA is off
> > or pwritev2() with RWF_DSYNC is not available.
> >
> > Support for disk writes with FUA is handled in qemu_pwritev_fua(),
> > which uses pwritev2() with RWF_DSYNC if available, otherwise falls
> > back to pwritev2() with no flags followed by flush using
> > handle_aiocb_flush().
> >
> > If pwritev2() is not implemented, then disk write in the linear FUA
> > will fallback to pwrite() + handle_aiocb_flush().
> >
> > Signed-off-by: Pinku Deb Nath <prantoran@gmail.com>
> >
> > ---
> >
> > v5:
> > - Use pwritev for unsupported OSes
> >
> > v4:
> > - Add fallback when qemu_pwritev_fua() returns ENOSYS
> > - Similar fallback was not added for handle_aiocb_rw_vector()
> > since there is a preadv_present check in handle_aiocb_rw()
> >
> > v3:
> > - Changed signature to add fd, iov, nr_iov
> > - Return -ENOSYS for non-Linux hosts
> >
> > v2:
> > - Moved handle_aiocb_flush() into qemu_pwritev_fua()
> > - In handle_aiocb_rw_linear(), iovec with iovcnt=1 is created
> > based on the assumption that there will be only one buffer
> > ---
> >  block/file-posix.c | 68 ++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 56 insertions(+), 12 deletions(-)
>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
>

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