Re: 3 disk RAID1?

["Kristleifur Daðason" <kristleifur@gmail.com>]

On Mon, Mar 8, 2010 at 10:11 PM, Michael Evans <mjevans1983@gmail.com> wrote:
> 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...

Good advice.

The contents of the manual didn't really stick for me until I'd
actually *done* the tasks I was trying to figure out. The beautiful
and classic Catch-22 of learning new things. So I somewhat
empathically suggest you obtain a virtual machine program and start up
a few disposable virtual machines. (VirtualBox is pretty good and also
free.) Try creating some RAID's on VM's and getting them to boot, try
failing a device and readding it, etc.

And keep the manual in a window next to the virtual machine's window :)

-- Kristleifur
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