On Dec 8, 2003, "ART KAGEL, BLOOMBERG/ 65E 55TH" <KAGEL@xxxxxxxxxxxxx> wrote: > If you have 10 drives or say 20GB each for 200GB RAID5 will use 20% > for parity (assuming you set it up as two 5 drive arrays) so you > will have 160GB of storage. And why would you set it up as two 5-drive arrays, instead of as a single 10-disk array, if the main point of RAID5 over is RAID10 is to maximize storage at the expense of significant performance degradation for writes and the risk of array corruption that you have described in your posting. Personally, if I wanted something that wouldn't go down, I wouldn't trust RAID10 much more than RAID5; I'd go with RAID15, i.e., RAID1 pairs that together create a RAID5 array. With RAID10, if you're unlucky enough that two disks fail at the same time (which is one of the main reasons to avoid RAID5 in your argument) and they happen to form a RAID1 pair, you lose everything. Sure, the probability that, given the failure of two disks, they're the disks in a single RAID1 set, is pretty small, but it's still not negligible. And, guess what: it's exactly at the time of recovery of the lost disk that the other disk of the pair will be under the most stress, and thus more likely to fail. With RAID15, you'd have to lose two complete pairs of disks to lose data. But it's all about tradeoffs. It's certainly important to describe the risks of any of the solutions, and you do have a good case against RAID5. But any hard-and-fast rules in this subject is bound to be wrong for certain scenarios. People have to be aware of the risks and, together with their budget and performance requirements, select what seems to be the best trade off for them. -- Alexandre Oliva Enjoy Guarana', see http://www.ic.unicamp.br/~oliva/ Red Hat GCC Developer aoliva@{redhat.com, gcc.gnu.org} CS PhD student at IC-Unicamp oliva@{lsd.ic.unicamp.br, gnu.org} Free Software Evangelist Professional serial bug killer
