On Thu, Mar 30, 2000 at 02:21:45PM -0600, Bill Carlson wrote:
> 1+5 would still fail on 2 drives if those 2 drives where both from the
> same RAID 1 set. The wasted space becomes more than N/2, but it might
> worth it for the HA aspect. RAID 6 looks cleaner, but that would require
> someone to write an implementation, whereas you could do RAID 15 (51?)
> now.
2 drives failing in either RAID 1+5 or 5+1 results in a still available
array:
minimal RAID 1+5 (ie: mirroring stripes)
stripe 1 mirrored stripe 2
sda1 sdd1
sdb1 sde1
sdc1 sdf1
If 2 in the same stripe die, then that stripe dies, but the array is still
there since the other stripe is fine. If 1 in each stripe die then both
stripes are still available (although degraded) and the whole array is still
up.
You can lose a third disk as well without any problems (either all 3 on one
side, or a 1/2 split which leaves the array available but degraded.)
minimal RAID 5+1 (ie: striped mirrors)
sda1 mirrored sdd1
sdb1 mirrored sde1
sdc1 mirrored sdf1
and then striped vertically. 2 disks failing in the same mirror means
the array goes into degraded mode, but it's still available. 2 disks
failing in different mirrors means that the array is still 100% up and
available (not degraded).
you can still lose a third disk, either both sides of a mirror and another,
or 1 from each mirror -- same result, the array is still available (and in
the latter case, non degraded).
In either case, losing a 4th disk could potentially bring the array down.
I'll agree, BTW, that this is a large amount of "wasted" space, but it
depends what your goals are. x2 disks may be worth it if you need a large
amount of reliability. I haven't looked at them too much, but since RAID
6/7(?) handle double disk failures, they're worth looking into.
> My thought here is leading to a distributed file system that is server
> independent, it seems something like that would solve a lot of problems
It would be pretty nifty.
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