Red Hat Bugzilla – Bug 234110
RAID5 on 2 disks
Last modified: 2007-11-30 17:12:00 EST
Description of problem:
I want to setup RAID5, not RAID1, on a pair of disks during install, but
anaconda insists that I must have 3 disks.
Version-Release number of selected component (if applicable):
Fedora Core 6
Steps to Reproduce:
1. Select RAID5 with 2 disks
2. It says you must have at least 3 devices to setup RAID5.
Setup RAID5 on 2 disks.
Yeah, RAID5 on 2 disks doesn't make much sense most of the time. But it has
one significant advantage over RAID1 on 2 disks: you can later extend the
RAID5 onto a 3rd disk using mdadm --grow, and get more usable space. You
can't do that with RAID1.
In my case, I have 2x400GB drives. That's enough space for me for the next
year or so, but I expect to outgrow it soon. I'd really like to be able to
add a 3rd drive when I need 800GB.
We don't support doing degraded RAIDs like this because people would expect to
get the benefit of the RAID and then file bugs when they didn't.
While I can see some use cases for setting up degraded RAIDs, that's not what
I was trying to propose.
RAID5 with 2 disks gives you the same redundancy as RAID1, but with the
ability to add disks later. 2 x 400GB drives, RAID5'ed gives you 400GB of
usable space, and if either fail, you're ok. Same as RAID1. But you can add
disks to a RAID5 with mdadm --grow. So I could add a third 400GB drive to the
2x400 RAID5, and my usable capacity will grow from 400GB to 800GB. I can't do
that with RAID1.
Where's your parity disk with only 2 drives? We cannot support a scenario like
this during installation. Closing as WONTFIX.
Created attachment 151341 [details]
Test case to demonstrate the viability of 2-disk RAID5
"Where's your parity disk with only 2 drives?"
Spread across the two drives, just like it's spread across three drives.
Using the same diagram style as the link you provided:
disk0 | disk1 | disk2
A1 B1 P1
A2 P2 C2
P3 B3 C3
disk0 | disk1
Please look at the test case I attached as a demonstration of the validity of
this configuration. If there is something wrong with the test case, please
point it out.
Test case requirements:
- must be run as root
- uses 200MB of disk space
- uses /dev/md0, /dev/loop0, and /dev/loop1
- takes 1 minute 20 seconds to run on my 2.6GHz notebook
I fail to see the advantage to RAID 5 on two disks. You mention the only
advantage you want is the ability to grow the array a year or so down the road
when you need 800GB of storage space. That's some serious planning going on,
but do you not forsee yourself reinstalling in that timeframe or upgrading or
even switching to new hardware? Maybe not, I dunno.
Also, the performance hit you take with RAID 5 vs. straight up mirroring when
you only have two drives seems like a waste of resources. For that year of time
before you need more space, why not running mirroring and take the best
performance you can get given the hardware you have?
By definition, RAID 5 with fewer than three disks is in a degraded state and
we're not going to support these scenarios during installation via anaconda. If
you absolutely need this for your environment, consider a kickstart file
tailored to your environment.