From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joel Becker Subject: Re: [PATCH 1/3] fs: Document the reflink(2) system call. Date: Tue, 5 May 2009 15:04:59 -0700 Message-ID: <20090505220459.GN7835@mail.oracle.com> References: <20090505010703.GA12731@shareable.org> <20090505071608.GB10258@mail.oracle.com> <20090505130114.GD17486@mit.edu> <20090505131907.GF25328@shareable.org> <1241530798.7244.65.camel@think.oraclecorp.com> <20090505153629.GB31100@shareable.org> <1241538102.7244.72.camel@think.oraclecorp.com> <20090505160313.GF31100@shareable.org> <4A00A634.4020008@hp.com> <20090505215711.GA7574@shareable.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: jim owens , Chris Mason , Theodore Tso , linux-fsdevel@vger.kernel.org, jmorris@namei.org, ocfs2-devel@oss.oracle.com, viro@zeniv.linux.org.uk To: Jamie Lokier Return-path: Received: from acsinet11.oracle.com ([141.146.126.233]:39465 "EHLO acsinet11.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753386AbZEEWGS (ORCPT ); Tue, 5 May 2009 18:06:18 -0400 Content-Disposition: inline In-Reply-To: <20090505215711.GA7574@shareable.org> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Tue, May 05, 2009 at 10:57:11PM +0100, Jamie Lokier wrote: > jim owens wrote: > > 3) the granularity of the COW (1-byte write may cause > > 1-block up through whole file copy) is fs-dependent. > > And yet ENOSYS if the fs cannot implement any COW, and it isn't > possible for userspace to duplicate the semantics by explicit copying? The point-in-time of the snapshot is what's important here. > Do we say anything about attribute changes triggering COW or not, or > leave it fs-dependent? Given 3) fs-dependent makes sense, but it's > nice to know in advance if { reflink -R old_tree saved_tree; chmod -R > a-w saved_tree } will be as expensive as copying or as cheap as linking. "Shares the data extents of the source file". I should hope that chmod doesn't require copying out all the data. Joel -- Life's Little Instruction Book #267 "Lie on your back and look at the stars." Joel Becker Principal Software Developer Oracle E-mail: joel.becker@oracle.com Phone: (650) 506-8127