Raid 5 (was: Mounting of Linux volumes)

Nicholas Leippe 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:

      RAID1
  RAID5    RAID5
a1 a2 a3  b1 b2 b3

- Yields only 1/3 capacity efficiency (only 2 of the 6 drives worth of
  data).
- 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.

       RAID0
     /   |   \
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


-- 
Respectfully,

Nicholas Leippe
Sales Team Automation, LLC
1335 West 1650 North, Suite C
Springville, UT  84663 +1 801.853.4090
http://www.salesteamautomation.com



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