Re: wish for Linux MD mirrored raid types

[Miles Fidelman <mfidelman@meetinghouse.net>]

Roberto Spadim wrote:
> hum, i asked this question one time, the point is:
> raid1 code is very easy
> raid10 code is more complex
>
> easy = faster, less memory, less cpu
> complex = faster?, more memory? more cpu?
>
> check others raid system (freebsd, netbsd) and check how they do...
>
> 2011/5/6 Keld Jørn Simonsen<keld@keldix.com>:
>    
>>> Keld Jørn Simonsen wrote:
>>>        
>>>>> As you say, RAID10,near on four disks is pretty much identical to
>>>>> RAID1+0 - i.e., a stripe of two normal RAID1 pairs.
>>>>>
>>>>>            

of course RAID10 offers the far option, and the option to run on an odd 
number of drives (I currently use it on a couple of 4-drive servers, but 
as I  look at some replacement hardware I've been thinking that RAID10 
on a 5-drive configuration offers a nice mix of reliability and performance)

re. easy/complex - I'm not sure I really believe that - when I've 
reviewed options, I come to the conclusion that:

RAID1 - wastes a lot of drive space, particularly if you want to 
maintain reliability after a single-drive failure (requires a minimum of 
3 mirrored drives)

RAID5 and RAID6 are better when it comes to mixing efficiency with 
multi-drive failures, but have a couple of odd failure modes and are a 
real pain to rebuild after a failure

RAID1+0 and RAID0+1 are interesting combination - but my sense is that 
every time you nest a layer, you're adding configuration complexity, 
processing delays (low level cpu cycles and i/o transactions), and just 
that much more complexity if you have to reconfigure or rebuild and 
array (particularly if you're running LVM and DRBD on top of the basic 
disk arrays, as I am)

md RAID10 chops out a layer of nesting and makes better use of disk 
space - by combining block replication and striping into a single layer 
- providing what, to me, is a good balance of disk use, multiple copies 
of data, performance, and managability - so far, its worked well for me



-- 
In theory, there is no difference between theory and practice.
In<fnord>  practice, there is.   .... Yogi Berra


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