On Thursday 07 June 2007 16:08:54 Greg Freemyer wrote: > On 6/6/07, Matthew Stringer <[EMAIL PROTECTED]> wrote: > > Hi, > > > > I've a server with 24 500GB SATA 2 hard drives with RAID5 running 10.2. > > *** Warning, the hp trained storage engineer part of me is coming out.**** > > A 24 disk raid5 is not normally recommended. It is much more likely > to fail than a raid50 where you are striping across several smaller > raid5's. > > Even more reliable is raid6. Personally I like raid10 for most > situations. raid10 is the most expensive, but the most reliable one > highest performing. > > As an example, assume you have a raid50 made of 6 4-disk raid5 sets > striped together. Assume the chance of a first disk failure in a year > is X. > Assume the chance of a second disk failure during the window of > vulnerability is Y. > > FYI: The window of vulnerability is however long it takes to replace a > failed drive and rebuild the array. On big arrays, just the rebuild > time can be a couple days. And if the array is both very large and in > extremely heavy use 24/7 it can be a couple weeks. > > Back to the example: > The overall failure likely hood is [(4 * X) * ((4-1) * Y)] * 6 or 72 * X * > Y. > > Hopefully you can figure out where the numbers came from. > > Now for a single 24 disk array the failure likelihood is: > (24 * X) * ((24 -1) * Y) or 552 * X * Y. > > That is the big raid5 array is roughly 7.5 times more likely to have a > total failure than a raid50 with the same number of drives would be. > > (Admittedly the raid50 holds only 75% of raw disk capacity and the > raid5 is going to hold a little more than 95%.) > > Given the price of drives, you might want to consider replacing all of > those 500GB drives with 1000GB drives and doing raid10. (about $500 > each I think). > > Then you have the same capacity and the failure likelyhood would be: > 24 * X * 1 * Y (ie any of the 24 can initially fail, but only its > partner failing causes a data loss). > > So your online data would be about 20 times safer for $12K. If I had > that much data, I would seriously consider what raid strategy I would > use, but I really doubt I would end up choosing a 24-disk raid5. > > Greg > -- > Greg Freemyer > The Norcross Group > Forensics for the 21st Century
All you say is true however, budget dictates, I needed >10TB of storage for £6K. You're suggesting spending another £12K on new drives alone! If I'd bought larger disks and used RAID50 I'd have half the disk I/O too which isn't desirable. In my case the data isn't that critical hence the solution I put in place, which incidentally with OpenSUSE 10.2 and XFS has been spot on! Matthew -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
