Dave Hitz wrote:
So at one parity disk per 14 the overhead is 1/14 = 7%, and at one parity disk per 26 the overhead is 3.8%.
Brian Rice brice@gnac.com writes:
The cost per usable gigabyte of my F630's has received *considerable* executive scrutiny recently. So I'm just a leeeetle sensitive about this kind of thing.
You *really* want to use one RAID group per 14 disks. Trust me.
Right now, we have 3 F630 systems (40 disk SCSI, 40 disk SCSI, and 26 disk FC). On our first F630, we have a 26 disk RAID group because multiple RAID groups weren't available at the time. On that RAID group, disk reconstructions seem to take forever. Once it took 23 hours, under only moderate load. I tried cranking the raid.reconstruct_speed dial higher (I think I had it set to 3), but then everyone starting getting "nfs server not responding" errors.
So, not only are you subject to greater risk during those 23 hours, but your users will complain about how slow everything is (even if the dial is set at 2 or 3). Let's face it, NetApps don't perform very well during disk reconstruction.
On a 14 disk RAID group on an F630, the slowest reconstruction we've finished in under 4 hours. The fastest was under 2 hours (on the F630 FC).
I guess it's important to note that disk reconstruction speed does not increase linearly. Double the disks in a RAID group and it can easily quadruple the time.
Dan