Re: [PLUG] Two-Drive Software Raid 5?

[Matt Simmons <standalone.sysadmin@gmail.com>]

I'll start out by saying that it's certainly...um,
unconventional...but if md will do it, then md will do it. I wouldn't
run it in production, just because it makes me feel dirty, but to each
their own.

On the other hand, you will be taking a performance hit with this over
RAID-1, because you still have to calculate parity. Probably not a
_big_ hit at this point, since you've only got two spindles, but it'll
be there.

I would be very interested to see how the md code treats 2 drive
RAID-5, as in, where does it put the parity bits. While you only have
2 disks, it doesn't matter, but when you add a third, I have to wonder
how it will assign parity.

I'm not sure it's any more conventional, but there is a methodology of
going from RAID-1 to RAID-5
(http://scott.wallace.sh/2007/04/14/converting-raid1-to-raid5-with-no-data-loss/),
although if you suspect you're going to wind up with more than 3-4
disks, I urge you to consider RAID-6 instead.

--Matt


On Mon, Jan 3, 2011 at 10:31 PM, Rich Freeman <r-plug@thefreemanclan.net> wrote:
> Has anybody had much experience with using linux software raid (mdadm)
> to set up a two-drive raid5?
>
> From what I've read the raid5 implementation in mdadm seems to handle
> this just fine - the parity of a single drive is essentially a mirror
> of that drive. ÂApparently you can overwrite the metadata on a raid1
> with raid5 metadata and then treat it as a raid5, or you can just
> create a raid5 initially.
>
> Has anybody used this in practice? ÂI need to replace a failed 120GB
> drive and since I'll obviously be stepping up in size I'll probably
> just migrate to a new array. ÂI'd like to start with two 1TB drives
> but leave room for future expansion. ÂExpanding a RAID1 is not
> practical, but expanding a RAID5 is trivial.
>
> There was an ubuntu forum discussion on this a few years ago with
> about 15 people posting "thou shalt not use 2 disks on raid5," one
> person explaining that it works fine but raid1 would still be
> recommended, and no real evidence that anybody did any serious testing
> of their assertions one way or another. ÂI figured I'd see if anybody
> here has tried it, as I'd rather not have 15 people send me emails
> explaining the difference between RAID1 and RAID5... Â I see no
> theoretical reason why RAID5 can't work with two drives - there is as
> much parity data as data-data so you should be able to re-create the
> one from the other regardless of what format it is in...
>
> Rich
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