Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
On Fri, 30 May 2008, Stefan Seifert wrote: > On Friday, 30. May 2008, drbd at bobich.net wrote: >> I think you'll find that you are. With RAID 51, you can lose up to 1 disk >> per side. With RAID 15 you could lose all disks on one side without even >> needing to fail over to the backup node. > > If you could lose all disks on one side in a RAID 15 without having to fail > over, why would you need to failover if one of the RAID 5s in a RAID 51 fails > due to two drives failing? That is a fair point, but with RAID 51 you can withstand much fewer disk failure combinations than with RAID 15. >> RAID isn't about speed, it's about fault tolerance, and RAID 15 is more >> fault tolerant than RAID 51. > > So in your RAID 15 you lose two hard drives of one node and before being able > to replace it the other node goes down because of a failing power supply or > whatever. Your cluster's down. > On RAID 51 you lose two hard drives of one node and then the other node goes > down. Your cluster's down, too. No difference here. You're throwing in PSU failures here which our out of scope for RAID. It's not a reasonable comparison. > I've played it through with many other cases. In each of them I get exactly > the same characteristics. The only difference is wether the RAID 1 or RAID 5 > fails first which makes no difference at all on the cluster's status. It does. With RAID 15 you can tolerate failures of 1/2 of the disks as long as you don't lose more than one mirror set alltogether. With RAID 51 you can tolerate failure of 1/2 of the disks only if they are all on the same machine - which won't happen because that machine will be down from the moment the 2nd disk fails. RAID 15 will yield better uptimes than RAID 51. If I have a moment I'll post an equation for it. Gordan