Re: RAID10 Layouts

[Bill Davidsen <davidsen@tmr.com>]

Keld Jørn Simonsen wrote:
> On Fri, Aug 21, 2009 at 06:43:28PM +0200, Goswin von Brederlow wrote:
>   
>> Info@quantum-sci.net writes:
>>
>>     
>>> Hello list,
>>>
>>> Researching RAID10, trying to learn the most advanced system for a 2
>>> SATA drive system.  Have two WD 2TB drives for a media computer, and
>>> the most important requirement is data redundancy.  I realize that
>>> RAID is no substitute for backups, but this is a backup for the
>>> backups and the purpose here is data safety.  The secondary goal is
>>> speed enhancement.  It appears that RAID10 can give both.
>>>
>>> First question is on layout of RAID10.  In studying the man pages it
>>> seems that Far mode gives 95% of the speed of RAID0, but with
>>> increased seek for writes.  And that Offset retains much of this
>>> benefit while increasing efficiency of writes.  What should be the
>>> preference, Far or Offset?  Are they equally as robust?
>>>       
>> All raid10 layouts offer the same robustness. Which layout is best for
>> you really depends on your use case. Probably the biggest factor will
>> be the average file size. My experience is that with large files the
>> far copies do not cost noticeable write speed while being twice as
>> fast reading as raid1.
>>     
>
> The file system elevator makes up for the Far write head movement.
>
>   
>>> How safe is the data in Far or Offset mode?  If a drive fails, will
>>> a complete, usable, bootable system exist on the other drive?
>>> (These two are the only drives in the system, which is Debian
>>> Testing, Debian kernel 2.6.30-5) Need I make any special Grub
>>> settings?
>>>       
>> I don't think lilo or grub1 can boot from raid10 at all with offset or
>> far copies. With near copies you are identical to a simple raid1 so
>> that would boot.
>>     
>
> there is a howto on setting up a system, that can continue runnig, if one 
> disk fails at
> http://linux-raid.osdl.org/index.php/Preventing_against_a_failing_disk
>
>   
>>> How does this look:
>>> # mdadm --create /dev/md0 --level=raid10 --layout=o2 --metadata=1.2 --chunk=64 --raid-disks=2 missing /dev/sdb1
>>>       
>> On partitions it is save to use 1.1 format. Saves you 4k. Jupey.
>>
>> You should play with the chunksize though and try with and without
>> bitmap and different bitmap sizes. Bitmap costs some write performance
>> but it greatly speeds up resyncs after a crash or temporary drive
>> failure.
>>     
>
> I would recommend a bigger chunk size. at least 256 kiB.
>   

You really want to look at stripe-size and stride-size creating an 
ext[234] filesystem on top of raid, good things to happen there.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc

"You are disgraced professional losers. And by the way, give us our money back."
    - Representative Earl Pomeroy,  Democrat of North Dakota
on the A.I.G. executives who were paid bonuses  after a federal bailout.



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