Commit 1cdc3239 authored by Denis V. Lunev's avatar Denis V. Lunev Committed by Kevin Wolf
Browse files

block: use fallocate(FALLOC_FL_PUNCH_HOLE) & fallocate(0) to write zeroes 



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.

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>
Signed-off-by: default avatarDenis V. Lunev <den@openvz.org>
Reviewed-by: default avatarMax Reitz <mreitz@redhat.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent d50d8222

block/raw-posix.c

+19 −0
Original line number Diff line number Diff line
@@ -967,6 +967,25 @@ static ssize_t handle_aiocb_write_zeroes(RawPosixAIOData *aiocb) 
    }

#endif



#ifdef CONFIG_FALLOCATE_PUNCH_HOLE

    if (s->has_discard && s->has_fallocate) {

        int ret = do_fallocate(s->fd,

                               FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,

                               aiocb->aio_offset, aiocb->aio_nbytes);

        if (ret == 0) {

            ret = do_fallocate(s->fd, 0, aiocb->aio_offset, aiocb->aio_nbytes);

            if (ret == 0 || ret != -ENOTSUP) {

                return ret;

            }

            s->has_fallocate = false;

        } else if (ret != -ENOTSUP) {

            return ret;

        } else {

            s->has_discard = false;

        }

    }

#endif



#ifdef CONFIG_FALLOCATE

    if (s->has_fallocate && aiocb->aio_offset >= bdrv_getlength(aiocb->bs)) {

        int ret = do_fallocate(s->fd, 0, aiocb->aio_offset, aiocb->aio_nbytes);