From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Chinner Subject: Re: [PATCH 0/5 v2] add extent status tree caching Date: Mon, 22 Jul 2013 11:38:31 +1000 Message-ID: <20130722013831.GE11674@dastard> References: <1373987883-4466-1-git-send-email-tytso@mit.edu> <51E8356C.9030603@redhat.com> <20130718185310.GA17548@thunk.org> <51E88ECD.3040806@redhat.com> <20130719025934.GE17938@thunk.org> <20130719033309.GQ11674@dastard> <20130719161930.GF17938@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Eric Sandeen , Ext4 Developers List , Zheng Liu To: Theodore Ts'o Return-path: Received: from ipmail04.adl6.internode.on.net ([150.101.137.141]:11484 "EHLO ipmail04.adl6.internode.on.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753922Ab3GVBif (ORCPT ); Sun, 21 Jul 2013 21:38:35 -0400 Content-Disposition: inline In-Reply-To: <20130719161930.GF17938@thunk.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Fri, Jul 19, 2013 at 12:19:30PM -0400, Theodore Ts'o wrote: > On Fri, Jul 19, 2013 at 01:33:09PM +1000, Dave Chinner wrote: > > An ioctl is kinda silly for this. Just use O_NONBLOCK when calling > > open() and do the prefetch right in the open call. The open() can > > block, anyway, and what you are trying to do is non-blocking IO with > > AIO, so it seems like we've already got a sensible, generic > > interface for triggering this sort of prefetch operation. > > O_NONBLOCK (either set via open or fcntl) is a possibility, since it's > carefully defined to be unspecified for regular files by SUSv3. It is > quite different from the existing semantics for O_NONBLOCK, though. > Currently, for all file types where O_NONBLOCK is not ignored, open(2) > is guaranteed itself not to block. If we use O_NONBLOCK for regular > files to mean that any necessary metadata blocks required for AIO to > be "A" will be cached, then it will make open(2) much more likely to > block. Also, for all file types where O_NONBLOCK is not ignored, > read(2) will not block but instead return -1 and set errno to EAGAIN. > This would also be a change. > > If we tried to get this new semantics for O_NONBLOCK to be accepted by > the Austin Group for standardization in the future, would they accept > it, or would they say, "this makes me vommit"? I have a suspicion > there reaction might be closer to the latter.... > > If we want a VFS-level API, in my opinion an fadvise() flag would be a > better choice. Sure. Make it an fadvise() flag - just don't add ioctls for things that are generically useful. On second thoughts - you're trying to get the extent map read in. We already have an interface for querying extent maps - fiemap. FIEMAP_FLAG_PREFETCH along with the range of the file you want the extent map prefetched for? Cheers, Dave. -- Dave Chinner david@fromorbit.com