Re: implications of partitioning and raid

[Robin Hill <robin@robinhill.me.uk>]

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On Thu Jan 05, 2012 at 08:40:34AM -0700, Peter W. Morreale wrote:

> 
> I'm wondering what the implications are for having multiple raid sets on
> a partitioned disk wrt to disk failures. 
> 
> For example, suppose I create two partitions on a set of disks and
> create raid sets on those partitions.  Further, not all raid sets
> reference the same disks.  IOW, md0 references disks 1 and 2, and md1
> references disk 1 and 3.  (overly simplistic for discussion purposes)
> 
> Assume a portion of disk 1 goes 'bad' (localized within one of those
> partitions), is noticed by md and a rebuild is warranted.  
> 
> What is the behavior?  
> 
Any failure will only affect the single array member where the error
occurred. In most cases this is due to a catastrophic disk failure
though, so will also occur on other partitions of the disk (and the
arrays using them) as well.

> Will both raid sets start a rebuild?  Or only the affected raid set?
> 
> IOW, would there be two rebuild tasks, one for each raid set?  Or a
> single rebuild that encompasses all raid sets (within the same raid
> level, of course) on the disk in question? 
> 
Rebuild processes are per raid set, but only one rebuild will be done at
a time using the same disk (so if disk 1 fails and is replaced, then
there's separate rebuilds pending for md0 and md1, but only one will run
at a time).

> What I am getting at is whether there would be any advantage to
> partitioning disks for failure purposes.   
> 
There's an advantage in being able to prioritise the rebuilds (so I can
rebuild the array containing / before that containing /opt for example)
and there's some cases where there's a transient failure (a write error
causes the array member to be failed, but testing shows up no errors) in
which case it reduces the rebuild time as only a single array needs to
be rebuilt. The latter is pretty rare though, unless you have other
underlying problems (such as power issues or poor quality I/O
chipsets/drivers).

HTH,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
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