From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: Some very basic questions Date: Wed, 22 Oct 2008 20:28:26 +0200 Message-ID: <48FF70CA.9090604@redhat.com> References: <20081021132322.271ad728.skraw@ithnet.com> <48FDD710.5050702@hp.com> <20081021190136.89b2c6af.skraw@ithnet.com> <20081021171513.GA8799@infradead.org> <48FE11F9.7040700@gmail.com> <20081022142759.ac33a16c.skraw@ithnet.com> <1224681345.6448.4.camel@think.oraclecorp.com> <48FF2A5B.80108@redhat.com> <48FF396B.1020700@redhat.com> <48FF3CB9.6070404@redhat.com> <48FF3EB8.6050306@redhat.com> <48FF4082.407@redhat.com> <48FF4302.5030204@redhat.com> <48FF45EE.7010001@redhat.com> <1224689639.6448.72.camel@think.oraclecorp.com> <48FF4A17.80204@redhat.com> <48FF4CAE.3080206@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Cc: Chris Mason , Stephan von Krawczynski , Christoph Hellwig , jim owens , linux-btrfs@vger.kernel.org To: Ric Wheeler Return-path: In-Reply-To: <48FF4CAE.3080206@redhat.com> List-ID: Ric Wheeler wrote: > For any given set of disks, you "just" need to do the math to compute > the utilized capacity, the expected rate of drive failure, the rebuild > time and then see whether you can recover from your first failure > before a 2nd disk dies. > Spare disks have the advantage of a fully linear access pattern (ignoring normal working load). Spare capacity has the advantage of utilizing all devices (if you have a hundred-disk fs, all surviving disks participate in the rebuild; whereas with spare disks you are limited to the surviving raidset members. Spare capacity also has the advantage that you don't need to rebuild free space. > In practice, this is not an academic question since drives do > occasionally fail in batches (and drives from the same batch get > stuffed into the same system). This seems to be orthogonal to the sparing method used; and in both cases the answer is to tolerate dual failures. File-based redundancy has the advantage here of allowing triple mirroring for metadata and frequently written files, and double parity raid for large files. > I suspect that what will be used in mission critical deployments will > be more conservative than what is used in less critical path systems That's true, unfortunately. But with time people will trust the newer, more efficient methods. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.