From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Mason Subject: Re: Data Deduplication with the help of an online filesystem check Date: Thu, 4 Jun 2009 07:43:57 -0400 Message-ID: <20090604114357.GK13945@think> References: <1240953977.15136.76.camel@think.oraclecorp.com> <20090428221455.GA27794@cip.informatik.uni-erlangen.de> <1240960687.15136.88.camel@think.oraclecorp.com> <20090429120300.GG22917@cip.informatik.uni-erlangen.de> <1241010875.20099.2.camel@think.oraclecorp.com> <20090429135804.GI22917@cip.informatik.uni-erlangen.de> <1241015512.20099.30.camel@think.oraclecorp.com> <20090429152614.GJ22917@cip.informatik.uni-erlangen.de> <1241019915.20099.35.camel@think.oraclecorp.com> <20090604084919.GB22607@cip.informatik.uni-erlangen.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Heinz-Josef Claes , Edward Shishkin , Tomasz Chmielewski , linux-btrfs@vger.kernel.org To: Thomas Glanzmann Return-path: In-Reply-To: <20090604084919.GB22607@cip.informatik.uni-erlangen.de> List-ID: On Thu, Jun 04, 2009 at 10:49:19AM +0200, Thomas Glanzmann wrote: > Hello Chris, > > > > My question is now, how often can a block in btrfs be refferenced? > > > The exact answer depends on if we are referencing it from a single > > file or from multiple files. But either way it is roughly 2^32. > > could you please explain to me what underlying datastructure is used to > monitor if the block is still referenced or already free? Is a counter > used, bitmap (but that can't be if is 2^32) or some sort of list? I > assume that a counter is used. If this is the case, I assume when a > snapshot for example is deleted the reference counter of every block > that was referenced in the snapshot will be decremented by one. Is this > correct or am I missing something here? It is a counter and a back reference. With Yan Zheng's new format work, the limit is not 2^64. When a snapshot is deleted, the btree is walked to efficiently drop the references on the blocks it referenced. >>From a dedup point of view, we'll want the dedup file to hold a reference on the file extents. The kernel ioctl side of things will take care of that part. -chris