Re: RAID6 Reshape Gone Awry

[Roman Mamedov <rm@romanrm.ru>]

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On Fri, 03 Aug 2012 15:37:36 +0200
David Brown <david.brown@hesbynett.no> wrote:

> That's not how I understand the disk layout - if I'm right, it is still 
> a monstrosity, but one that will offer protection on disk failure.
> 
> As I read it, he has this (prior to adding the new disk):
> 
> md0 = raid6(sda5, sdb5, sdc5, sdd5, sde5)
> md1 = raid6(sda6, sdb6, sdc6, sdd6, sde6)
> ...
> md9 = raid6(sda14, sdb14, sdc14, sdd14, sde14)
> 
> If that's the case, then it will be an administrative mess (as the OP is 
> now experiencing), but it will protect the data, and if the LVM is a 
> linear concatenation of these then performance normally will be okay. 

If you want the RAID5/6 write performance to be okay, you will want to
increase stripe_cache_size to a good value [1] -- and that's per array, and the
RAM consumption increases linearly with disk count - so on 10 five-member
arrays you won't have anywhere near enough RAM to have a sufficient
stripe_cache on all of them.

In other words, one more aspect in which this multi-array configuration is
highly suboptimal. :)

[1]
http://peterkieser.com/2009/11/29/raid-mdraid-stripe_cache_size-vs-write-transfer/

-- 
With respect,
Roman

~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Stallman had a printer,
with code he could not see.
So he began to tinker,
and set the software free."

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