From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: fs corruption exposed by "xfs: increase prealloc size to double that of the previous extent" Date: Mon, 17 Mar 2014 01:38:00 +0000 Message-ID: <20140317013759.GV18016@ZenIV.linux.org.uk> References: <20140315210216.GP18016@ZenIV.linux.org.uk> <20140317001130.GA7072@dastard> <20140317002918.GT18016@ZenIV.linux.org.uk> <20140317012804.GU18016@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-fsdevel@vger.kernel.org, Brian Foster , xfs@oss.sgi.com, Dave Chinner To: Dave Chinner Return-path: Content-Disposition: inline In-Reply-To: <20140317012804.GU18016@ZenIV.linux.org.uk> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: xfs-bounces@oss.sgi.com Sender: xfs-bounces@oss.sgi.com List-Id: linux-fsdevel.vger.kernel.org On Mon, Mar 17, 2014 at 01:28:04AM +0000, Al Viro wrote: > but that is unsafe - consider a situation when you are writing 20Kb from e.g. > 0.5Kb offset from the beginning of last (4Kb) block. You have 6 blocks > affected, right? One old, five new. And you want the last half-kilobyte s/the last/all but the first/, sorry. Basically, we want to get from OOOOOOOO (8 sectors of old data) to ONNNNNNN|NNNNNNNN|NNNNNNNN|NNNNNNNN|NNNNNNNN|NZZZZZZZ (1 sector of old data, 40 sectors of new data, 7 sectors of zeroes). We want the last 7 sectors zeroed out, but we do *not* want that to happen to the one sector of old data. OTOH, if the file had been 4K shorter (and all blocks had been new) we would want ZNNNNNNN|NNNNNNNN|NNNNNNNN|NNNNNNNN|NNNNNNNN|NZZZZZZZ. IOW, it's not just the last block we want to know about. There's simply not enough bandwidth in that interface to pass the information we would need for such mixed block runs... _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs