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Re: [linux-lvm] Volume alignment over RAID

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!

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

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 comedia it
        Communication Media & Services S.r.l.
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