Ken Burner wrote: > We're thinking of installing RAID arrays to replace JBOD collections >on our central AFS cell. <...snip...> A couple years ago, when we started averaging 250-300 user volumes per spindle, we did the calculations about how many tapes we would have to spin if one of those spindles ever failed, and...well...it was bad. Our primary concern, therefore, was fault tolerance, not performance. However, imnsho, RAID in this type of application will always take a performance hit. Now, before you all flame me for this, hear the reasons why. I did some performance benchmarks (http://www.cs.wisc.edu/~thomas/afs/disk-data.html), which, while not exhaustive, put the RAID boxes we purchased on a par with jbod disks we bought about the same time. Where I believe you take the performance hit is that with jbod we had 7 * 4Gig disks, with each disk seeking, reading, writing, etc., independently. With RAID, we have 4 * 9Gig disks (+1 parity disk), with _all_ the disks involved in _all_ the accesses. Now, depending on how well the fileserver can parallelize disks accesses (not well now, but stay tuned, it's supposed to get better), this may or may not be a problem, but in a highly parallel environment, you can get alot more work done on jbod than you can on RAID. So, remember that for total throughput, you are comparing your RAID array to a single disk, not your jbod array. That being said, I'm glad we made the move to RAID for some of our data. We have given our users the choice of paying the premium for RAID space or staying on jbod for non-home-directory space that they 'buy'. So far, no one has bought RAID space. Our backup system is very good, and we haven't had any problems going this way. You lose a day's changes, and so you have to know that you can live with that. This philosophy may or may not work in your environment. Of course, given this, I don't think RAID makes sense for use with replicated data, because with replicated data you already have fault tolerance. -- David Thompson <[EMAIL PROTECTED]> Associate Researcher Department of Computer Science University of Wisconsin-Madison http://www.cs.wisc.edu/~thomas 1210 West Dayton Street Phone: (608)-262-1017 Madison, WI 53706-1685 Fax: (608)-262-6626 --
