On 2015-01-27 at 08:51, 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 <d...@openvz.org> CC: Kevin Wolf <kw...@redhat.com> CC: Stefan Hajnoczi <stefa...@redhat.com> CC: Peter Lieven <p...@kamp.de> CC: Fam Zheng <f...@redhat.com> --- block/raw-posix.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/block/raw-posix.c b/block/raw-posix.c index 3c35b2f..c039bef 100644 --- a/block/raw-posix.c +++ b/block/raw-posix.c @@ -967,6 +967,20 @@ static ssize_t handle_aiocb_write_zeroes(RawPosixAIOData *aiocb) } #endif+#ifdef CONFIG_FALLOCATE_PUNCH_HOLE+ if (s->has_discard) { + 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); + } +#endif +
Sharing "has_discard" with handle_aiocb_discard() looks fine to me, because it's used for the the same do_fallocate() call there.
Once again, you should not abort if the first do_fallocate() returns ENOTSUP, because this is inconsistent with the behavior on the second call to handle_aiocb_write_zeroes() (where it falls through due to has_discard being false). Once again, this doesn't make a difference now, but very well might after the next patch.
And finally, do we need another has_foo for the fallocate(0) call? (like just "has_fallocate")
Max
