From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59973) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YGAFy-0008Rp-2j for qemu-devel@nongnu.org; Tue, 27 Jan 2015 12:48:55 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YGAFs-0003Td-TS for qemu-devel@nongnu.org; Tue, 27 Jan 2015 12:48:54 -0500 Received: from mx1.redhat.com ([209.132.183.28]:39540) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YGAFs-0003TQ-Km for qemu-devel@nongnu.org; Tue, 27 Jan 2015 12:48:48 -0500 Message-ID: <54C7CF75.9050502@redhat.com> Date: Tue, 27 Jan 2015 12:48:37 -0500 From: Max Reitz MIME-Version: 1.0 References: <1422366699-17473-1-git-send-email-den@openvz.org> <1422366699-17473-6-git-send-email-den@openvz.org> In-Reply-To: <1422366699-17473-6-git-send-email-den@openvz.org> Content-Type: text/plain; charset=iso-8859-15; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 5/7] block: use fallocate(FALLOC_FL_PUNCH_HOLE) & fallocate(0) to write zeroes List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Denis V. Lunev" Cc: Kevin Wolf , Peter Lieven , Fam Zheng , qemu-devel@nongnu.org, Stefan Hajnoczi 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 > CC: Kevin Wolf > CC: Stefan Hajnoczi > CC: Peter Lieven > CC: Fam Zheng > --- > 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