From: David Chinner Subject: Re: [RFC] add FIEMAP ioctl to efficiently map file allocation Date: Wed, 2 May 2007 00:20:49 +1000 Message-ID: <20070501142049.GG77450368@melbourne.sgi.com> References: <20070412110550.GM5967@schatzie.adilger.int> <20070416112252.GJ48531920@melbourne.sgi.com> <20070419002139.GK5967@schatzie.adilger.int> <20070419015426.GM48531920@melbourne.sgi.com> <20070430224401.GX5967@schatzie.adilger.int> <20070501042254.GD77450368@melbourne.sgi.com> <1177994346.3362.5.camel@entropy> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: David Chinner , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, xfs@oss.sgi.com, hch@infradead.org To: Nicholas Miell Return-path: Content-Disposition: inline In-Reply-To: <1177994346.3362.5.camel@entropy> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Mon, Apr 30, 2007 at 09:39:06PM -0700, Nicholas Miell wrote: > On Tue, 2007-05-01 at 14:22 +1000, David Chinner wrote: > > On Mon, Apr 30, 2007 at 04:44:01PM -0600, Andreas Dilger wrote: > > > This is actually for future use. Any flags that are added into this > > > range must be understood by both sides or it should be considered an > > > error. Flags outside the FIEMAP_FLAG_INCOMPAT do not necessarily need > > > to be supported. If it turns out that 8 bits is too small a range for > > > INCOMPAT flags, then we can make 0x01000000 an incompat flag that means > > > e.g. 0x00ff0000 are also incompat flags also. > > > > Ah, ok. So it's not really a set of "compatibility" flags, it's more a > > "compulsory" set. Under those terms, i don't really see why this is > > necessary - either the filesystem will understand the flags or it will > > return EINVAL or ignore them... > > > > > I'm assuming that all flags that will be in the original FIEMAP proposal > > > will be understood by the implementations. Most filesystems can safely > > > ignore FLAG_HSM_READ, for example, since they don't support HSM, and for > > > that matter FLAG_SYNC is probably moot for most filesystems also because > > > they do block allocation at preprw time. > > > > Exactly my point - so why do we really need to encode a compulsory set of > > > > Because flags have meaning, independent of whether or not the filesystem > understands them. And if the filesystem chooses to ignore critically > important flags (instead of returning EINVAL), bad things may happen. > > So, either the filesystem will understand the flag or iff the unknown flag > is in the incompat set, it will return EINVAL or else the unknown flag will > be safely ignored. My point was that there is a difference between specification and implementation - if the specification says something is compulsory, then they must be implemented in the filesystem. This is easy enough to ensure by code review - we don't need additional interface complexity for this.... Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group