[linux-lvm] Volume alignment over RAID

[Luca Berra]

On Fri, May 21, 2010 at 11:50:54AM -0700, Linda A. Walsh wrote:
> Lyn Rees wrote:
>>   > 192.00K is listed as the start of each!  GRR...why would that
>>>  be a default...I suppose it works for someone, but it's NOT a power of 2!
>>>  Hmph!
>>
>> 192 is a multiplier of 64... so it's aligned - assuming you used the whole 
>> disk as a PV (you didn't partition the thing first).
it is chunk aligned, not stripe aligned, reads would be ok, but
writes...

> Isn't 64 the amount written / disk, so the strip size is 256K?
> Wouldn't that make each strip have 1 64K chunk written odd,
> and the next 3 written in the next 'row'....
> I suppose maybe it doesn't matter...but when you break the pv up into
> vg's and lvs, somehow it seems odd to have them all skewed by 64K...
it will cause multiple R-M-W cycles fro writes that cross stripe
boundary, not good.

> Anyway...I wanted to redo the array anyway.  I didn't like the performance
> I was getting, so thought I'd try RAID 50.  I was only getting 150-300 on
> writes/reads on the RAID60 which seemed a bit low.  I get more than that
> on a a 4-data-disk RAID5 (200/400). It's a bit of pain to do all this 
> reconfiguring now, but better now than when they are all full!  It was
> a mistake to do RAID60, though I don't know if the performance on a 
> 10data-disk RAID6 would be any better for writes...still has to do
> alot of XORing even with a hardware card.
the choice between raid5 and raid6 has a lot to do with data safety.
also other constraints would mandate the use of spare drives in the
raid5 case. personally i prefer striping smaller redundant sets for
critical data. not to say that 10 is not a power of 2 and aligning lvm
becomes interesting.

> I had 2x6 and am going to try 4x3disks, so my hmmm....I guess now that I 
> think about it my strip size was really 8, not 4, since I had 2 of them.
yes it was 8
> But I'll still have a strip width of 8 with 4x3 RAID5's.  I don't know if it
> will be much faster or not...but guess I'll see.

-- 
Luca Berra -- bluca at comedia.it
         Communication Media & Services S.r.l.
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