Re: [Qemu-devel] [PATCH 2/2] block: use fallocate(FALLOC_FL_PUNCH_HOLE) & fallocate(0) to write zeroes

[Roman Kagan]

On Thu, Dec 25, 2014 at 08:37:30AM +0300, Denis V. Lunev wrote:
> This sequence works efficiently if FALLOC_FL_ZERO_RANGE is not supported.
> The idea is that FALLOC_FL_PUNCH_HOLE could not increase file size
> but it cleans already allocated blocks inside the file. If we have to
> create something new, simple fallocate will do the job.
> 
> This should increase performance a bit for not-so-modern kernels or for
> filesystems which do not support FALLOC_FL_ZERO_RANGE.
> 
> Signed-off-by: Denis V. Lunev <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> ---
>  block/raw-posix.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index 9e66cb7..60972a1 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -930,6 +930,18 @@ static ssize_t handle_aiocb_write_zeroes(RawPosixAIOData 
> *aiocb)
>  
>          ret = -errno;
>  #endif
> +#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
> +        do {
> +            if (fallocate(s->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> +                          aiocb->aio_offset, aiocb->aio_nbytes) == 0 &&
> +                fallocate(s->fd, 0,
> +                          aiocb->aio_offset, aiocb->aio_nbytes) == 0) {
> +                return 0;
> +            }
> +        } while (errno == EINTR);
> +
> +        ret = -errno;
> +#endif

This is suboptimal in that fallocate(FALLOC_FL_ZERO_RANGE) would always
be called in vain for such systems.  Might be worth another flag in
BDRVRawState?

Roman.