dan wrote: > > I would argue that file system stability is paramount for backups and > performance is really a distant second. who cares how fast the system
If you can't perform your backups in a 24-hour period, performance becomes more important. > is if it looses your data. Also, Raid5 has been extensively tested for > many years and it is VERY well know that there is a large performance > penalty for the parity calculation. *SOME* hardware raid cards can > minimize this but in software raid on linux it is a big deal. raid5 > will be substantially slower than raid10 or raid1 that have no parity > calculation. He is using a hardware RAID card. > That being said, if raid5(or6) is fast enough for you it is a mature and > stable option and a good choice, but certainly comes with a performance > penalty. You make it sounds like RAID-5 is incapable of saturating drive bandwidth. I haven't seen this on any modern (2-yr-old or newer) machine with more than one CPU. And he's using hardware RAID so the point is moot anyway. > examples: > raid10 with 6 drives in a raid0(r1-1+2, r1-3+4, r1-5+6), is 3 active > spindles because the other three are mirrors but has a worst case safety > of just 1 drive > raid10 in raid0(raid1-1+2+3, raid1-4+5+6) is just 2 spindles but is more > resilient because you can loose 2 or more drives and keep the array up. I think you have it backwards. A stripe of three mirrors (your first example) means that you could lose up to three drives and still have data (as long as you're not losing both drives in a mirror). > These are round numbers, kind of a rule of thumb. 6 volumes is about > where raid5 actually catches up. with a 4 drive set the raid5 penalty > brings is to 2 active spindles and has a large 33% latency penalty > because the array has to wait for all writes to complete while a raid10 > is 2 active spindles without a latency hit. I think you're overestimating the performance penalty of latency. This is worked around in most systems with caching (including a large amount of RAM on hardware controller cards), and only for writes. > raid6 shines with 10 or 12 spindles. I say raid 6 because I wouldnt > risk a large array to a single drive fault and a hotspare has a rebuild > window that makes me nervous. raid5 likes odd numbers of drives > active(not including hotspare) and raid6 like even numbers. I cant give > a scientific explanation and can only explain it is a phenomenon. It is explainable if you know how many columns are in your RAID setup. Number of drives isn't important; an integral multiple of drives compared to the number of columns is. -- Jim Leonard (trix...@oldskool.org) http://www.oldskool.org/ Help our electronic games project: http://www.mobygames.com/ Or check out some trippy MindCandy at http://www.mindcandydvd.com/ A child borne of the home computer wars: http://trixter.wordpress.com/ ------------------------------------------------------------------------------ Come build with us! The BlackBerry® Developer Conference in SF, CA is the only developer event you need to attend this year. Jumpstart your developing skills, take BlackBerry mobile applications to market and stay ahead of the curve. Join us from November 9-12, 2009. Register now! http://p.sf.net/sfu/devconf _______________________________________________ BackupPC-users mailing list BackupPC-users@lists.sourceforge.net List: https://lists.sourceforge.net/lists/listinfo/backuppc-users Wiki: http://backuppc.wiki.sourceforge.net Project: http://backuppc.sourceforge.net/
