On 02/02/2011 15:47, Robin Hill wrote:
On Wed Feb 02, 2011 at 09:21:20PM +0700, hansbkk@xxxxxxxxx wrote:
On Wed, Feb 2, 2011 at 6:36 AM, Roman Mamedov<rm@xxxxxxxxxx> wrote:
I have a RAID5 setup with 15 drives.
Looks like you got the problem you were so desperately asking for, with this
crazy setup. :(
Please give some more details as to what's so crazy about this.
Just the number of drives in a single RAID5 array I think. I'd be
looking at RAID6 well before I got to 10 drives.
I would think RAID6 would have made more sense, possibly with an
additional spare if these are large drives (over a few hundred GB?)
With 15, RAID6 + spare would probably be what I'd go with (depending on
drive size of course, and whether you have cold spares handy). For very
large drives, multiple arrays would be safer.
Or is there an upper limit as to the number of drives that's advisable
for any array?
I'm sure there's advice out there on this one - probably a recommended
minimum percentage of capacity used for redundancy. I've not looked
though - I tend to go with gut feeling& err on the side of caution.
If so, then what do people reckon a reasonable limit should be for a
RAID6 made up of 2TB drives?
As the drive capacities go up, you need to be thinking more carefully
about redundancy - with a 2TB drive, your rebuild time is probably over
a day. Rebuild also tends to put more load on drives than normal, so is
more likely to cause a secondary (or even tertiary) failure. I'd be
looking at RAID6 regardless, and throwing in a hot spare if there's more
than 5 data drives. If there's more than 10 then I'd be going with
multiple arrays.
If the load due to rebuild is a problem, it can make sense to split the
raid into parts. If you've got the money, you can start with a set of
raid1 pairs and then build raid5 (or even raid6) on top of that. With
raid 1 + 5, you can survive any 3 drive failures, and generally more
than that unless you are very unlucky in the combinations. However,
rebuilds are very fast - they are just a direct copy from one disk to
its neighbour, and thus are less of a load on the rest of the system.
Of course, there is a cost - if you have 15 2TB drives, with one being a
warm spare shared amongst the raid1 pairs, you have only 6 x 2TB storage.
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