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

[Stefan Hajnoczi <stefanha@redhat.com>]

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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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