Re: [Qemu-devel] [PATCH 6/8] block: use fallocate(FALLOC_FL_PUNCH_HOLE) & fallocate(0) to write zeroes

[Fam Zheng <famz@redhat.com>]

On Tue, 12/30 12:20, Denis V. Lunev wrote:
> This sequence works efficiently if FALLOC_FL_ZERO_RANGE is not supported.
> 
> Simple fallocate(0) will extend file with zeroes when appropriate in the
> middle of the file if there is a hole there and at the end of the file.
> Unfortunately fallocate(0) does not drop the content of the file if
> there is a data on this offset. Therefore to make the situation consistent
> we should drop the data beforehand. This is done using FALLOC_FL_PUNCH_HOLE
> 
> This should increase the 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 <den@openvz.org>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Peter Lieven <pl@kamp.de>
> ---
>  block/raw-posix.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index 7866d31..96a8678 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -968,6 +968,23 @@ static ssize_t handle_aiocb_write_zeroes(RawPosixAIOData *aiocb)
>  #endif
>  
>      s->has_write_zeroes = false;
> +
> +#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
> +    if (s->has_discard) {
> +        int ret;
> +        ret = do_fallocate(s->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> +                           aiocb->aio_offset, aiocb->aio_nbytes);
> +        if (ret < 0) {
> +            if (ret == -ENOTSUP) {
> +                s->has_discard = false;
> +            }
> +            return ret;
> +        }
> +        return do_fallocate(s->fd, 0, aiocb->aio_offset, aiocb->aio_nbytes);

Why is fallocate(0) necessary here? The manpage says:

Deallocating file space
	Specifying the FALLOC_FL_PUNCH_HOLE flag (available since Linux 2.6.38)
	in mode deallocates space (i.e., creates a hole) in the byte range
	starting at offset and continuing for len bytes.  Within the specified
	range, partial file system blocks are zeroed, and whole file system
	blocks are removed from the file.  After a successful call, subsequent
	reads from this range will return zeroes.

So the data are already zeroes after FALLOC_FL_PUNCH_HOLE.

Fam

> +    }
> +#endif
> +
> +    s->has_discard = false;
>      return -ENOTSUP;
>  }
>  
> -- 
> 1.9.1
> 
>