[linux-lvm] Volume alignment over RAID

Linda A. Walsh lvm at tlinx.org
Fri May 21 18:50:54 UTC 2010

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).
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...

But I haven't worked with RAIDS that much, so it's probably just a 
conceptual thing in my head.

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.

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.
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.


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