Rich Freeman on 3 Jan 2011 19:31:18 -0800

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[PLUG] Two-Drive Software Raid 5?

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.  Apparently 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?  I 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.  I'd like to start with two 1TB drives
but leave room for future expansion.  Expanding 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.  I 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...   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...

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