Re: RAID for large disks

[Luke S Crawford <lsc@prgmr.com>]

"Mag Gam" <magawake@gmail.com> writes:
> I want RAID 5 but without mirroring. The data is important but not that
> important.

Ok, there are performance advantages and disadvantages to RAID5.  
First, the advantage: reading is awesome.   almost as good as a stripe.

the other advantage:  writes in full stripe-size increments  are also pretty 
good... it's as good as a stripe minus the calculation overhead (calculation
overhead is pretty low these days, especially if you have a hardware 
raid)  


The downside:

writes less than the stripe size suck.  the thing is, each (16K in your
case) stripe needs to be calculated as one unit so the parity stuff works.

so if you write, say, 8K, it needs to read in the 8K of the stripe you are
not writing, combine that with the 8K you are writing, recalculate parity
and write the full 16K stripe. 

(write-back cache can solve this problem;  but write-back cache,
in most cases, is also pretty dangerous.  as another poster said, make
sure your raid card has battery backed cache before enabling write-back
cache)  

So if your stripe size is larger than your average write, things suck
mightily.

> If the controller creates a stripe size of 16k, do I need to do anything
> special with physical extends (in pvcreate or vgcreate) ?
> Do I need to do anything specific when creating a LV? I plan on striping my
> LV to create extra spindles. Do I need to create my ext3 filesystem with any
> particular settings? I am looking for a optimal tuning guide with emphasis
> on performace versus redudancy.

'striping my LV to create extra spindles'  -  I don't understand.  
the performance-enhancing spindles are physical disks... partitioning 
a disk and pretending that each partition is a physical disk isn't going
to help you.