Re: 3 disk RAID1?

[Michael Evans <mjevans1983@gmail.com>]

On Mon, Mar 8, 2010 at 1:04 PM, Mark Knecht <markknecht@gmail.com> wrote:
> On Mon, Mar 8, 2010 at 12:58 PM, Robin Hill <robin@robinhill.me.uk> wrote:
>> On Mon Mar 08, 2010 at 12:39:26PM -0800, Mark Knecht wrote:
>>
>>> Hi all,
>>>    I'm still very much on a steep learning curve about what I can do
>>> with Linux software RAID. In another thread this weekend a couple of
>>> responders discussed among themselves 3-disk RAID1 solutions that can
>>> survive if 2 disks die. I don't understand what that means. Can
>>> someone point me at a quick explanation? Is that really possible?
>>>
>>>    In general I'm using a few Wikipedia pages and gravitate toward the
>>> diagrams as much as anything.
>>>
>>> http://en.wikipedia.org/wiki/RAID
>>> http://en.wikipedia.org/wiki/RAID1#RAID_1
>>>
>>> RAID0 - striping, speed not reliability (2 disk minimum)
>>> RAID1 - duplicate data, no other protection (2 disk minimum)
>>>
>>>    How do I build RAID1 using three drives? Just duplicate the data 3
>>> times? If drives start going bad how do I determine which one or two
>>> are failing? (fsck? SMART?) With 3 drives 1 fail seems relatively
>>> straightforward to figure out, but 2?
>>>
>> A 3-disk RAID1 is just 3 duplicate copies, yes.  And RAID only protects
>> against hardware failures, so you know which disk has failed because it
>> gets kicked out of the array as faulty.  This is the same regardless of
>> how many mirrored copies you have (md will detect a write failure to a
>> drive and mark it as faulty - read errors will cause the failed block to
>> get rewritten).
>>
>> As for how to create it - it's just the same process as for a 2-disk
>> RAID1 but specifying 3 drives (assuming you're using Linux md software
>> RAID - if not, please specify what you're intending to use).  The manual
>> page for mdadm should give you everything you need - do ask if there's
>> anything you want clarifying though.
>>
>> Cheers,
>>    Robin
>
> Thanks Robin. Maybe I am getting smarter about this if I'm figuring
> out what others are talking about! ;-)
>
> Cheers,
> Mark
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When in doubt, read the manual two or three more times.

This might also help you: http://wiki.tldp.org/LVM-on-RAID  I wrote
some background comparison sections when I made that...
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