Re: [Qemu-devel] [PATCH 6/7] block: use fallocate(FALLOC_FL_PUNCH_HOLE) & fallocate(0) to write zeroes

[Max Reitz <mreitz@redhat.com>]

On 2015-01-30 at 03:42, Denis V. Lunev wrote:
> This sequence works efficiently if FALLOC_FL_ZERO_RANGE is not supported.
> Unfortunately, FALLOC_FL_ZERO_RANGE is supported on really modern systems
> and only for a couple of filesystems. FALLOC_FL_PUNCH_HOLE is much more
> mature.
>
> The sequence of 2 operations FALLOC_FL_PUNCH_HOLE and 0 is necessary due
> to the following reasons:
> - FALLOC_FL_PUNCH_HOLE creates a hole in the file, the file becomes
>    sparse. In order to retain original functionality we must allocate
>    disk space afterwards. This is done using fallocate(0) call
> - fallocate(0) without preceeding FALLOC_FL_PUNCH_HOLE will do nothing
>    if called above already allocated areas of the file, i.e. the content
>    will not be zeroed
>
> This should increase the performance a bit for not-so-modern kernels.
>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Max Reitz <mreitz@redhat.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Peter Lieven <pl@kamp.de>
> CC: Fam Zheng <famz@redhat.com>
> ---
>   block/raw-posix.c | 19 +++++++++++++++++++
>   1 file changed, 19 insertions(+)

Reviewed-by: Max Reitz <mreitz@redhat.com>