From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Xin Zhao" Subject: Re: how do versioning filesystems take snapshot of opened files? Date: Tue, 3 Jul 2007 13:15:06 -0400 Message-ID: <4ae3c140707031015y6f4a461vda7c683fd8f969ce@mail.gmail.com> References: <20070703090946.49e6062f@think.oraclecorp.com> <4ae3c140707030931u14119061m5d5dc69834292847@mail.gmail.com> <20070703130436.4a2f2b2c@think.oraclecorp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: "Bryan Henderson" , linux-fsdevel To: "Chris Mason" Return-path: Received: from wa-out-1112.google.com ([209.85.146.183]:1373 "EHLO wa-out-1112.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754556AbXGCRPH (ORCPT ); Tue, 3 Jul 2007 13:15:07 -0400 Received: by wa-out-1112.google.com with SMTP id v27so2845334wah for ; Tue, 03 Jul 2007 10:15:07 -0700 (PDT) In-Reply-To: <20070703130436.4a2f2b2c@think.oraclecorp.com> Content-Disposition: inline Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org OK. From discussion above, can we reach a conclusion: from the application perspective, it is very hard, if not impossible, to take a transactional consistent snapshot without the help from applications? Chris, you mentioned that "Many different applications support some form of pausing in order to facilitate live backups. " Can you provide some examples? I mean popular apps. Finally, if we back up a little bit, say, we don't care the transaction level consistency ( a transaction that open/close many times), but we want a open/close consistency in snapshots. That is, a file in a snapshot must be in a single version, but it can be in a middle state of a transaction. Can we do that? Pausing apps itself does not solve this problem, because a file could be already opened and in the middle of write. As I mentioned earlier, some systems can backup old data every time new data is written, but I suspect that this will impact the system performance quite a bit. Any idea about that? Thanks. On 7/3/07, Chris Mason wrote: > On Tue, 3 Jul 2007 12:31:49 -0400 > "Xin Zhao" wrote: > > > That's a good point! > > > > But this sounds hopeless to take a real consistent snapshot from app > > perspective unless you shutdown the computer. Right? > > Many different applications support some form of pausing in order > to facilitate live backups. You just have to keep it all in mind when > designing the total backup solution. > > -chris >