Raid 5 (was: Mounting of Linux volumes)
nick at byu.edu
Thu Dec 1 20:34:42 MST 2005
On Thursday 01 December 2005 06:45 pm, Michael Torrie wrote:
> My main file server has two RAID 5 arrays mirrored together in a RAID 1
> configuration. I feel fairly comfortable with that arrangement.
Except for the ability to perform the sync method that you described, I see
no good reason to do a raid 1 on top of raid 5. Economically it's worse
than raid 0+1, and there are fewer possible surviveable failure scenarios.
With the same number of drives, RAID 0 across several RAID 1 mirrors
achieves the same redundancy, more capacity, and many more survivable
failure scenarios, eg:
a1 a2 a3 b1 b2 b3
- Yields only 1/3 capacity efficiency (only 2 of the 6 drives worth of
- Is fully redundant, and adds additional 'partial redundancy'.
- A good controller can utilize all of the platters for reads, but with
RAID5 that depends on the dataset. At a minimum it will be reading
from only two platters simultaneously.
- Will write slower than a single RAID5 array.
/ | \
RAID1 RAID1 RAID1
a1 a2 b1 b2 c1 c2
- Yields 1/2 capacity efficiency
- Is fully redundant
- Will read from 3 platters simultaneously at a minimum, and can much
more easily utilize all 6 platters for reads (with a good hw controller,
sw controllers fail to read from RAID1 with an efficiency better than
a single drive)
- Should write hardly slower than a single RAID1 array
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