One Large md or Many Smaller md for Better Peformance?

[Moshe Yudkowsky <moshe@pobox.com>]

Question: with the same number of physical drives,  do I get better 
performance with one large md-based drive, or do I get better 
performance if I have several smaller md-based drives?

Situation: dual CPU, 4 drives (which I will set up as RAID-1 after being 
terrorized by the anti-RAID-5 polemics included in the Debian distro of 
mdadm).

I've two choices:

1. Allocate all the drive space into a single large partition, place 
into a single RAID array (either 10 or 1 + LVM, a separate question).

2. Allocate each drive into several smaller partitions. Make each set of 
smaller partitions into a separate RAID 1 array and use separate RAID md 
drives for the various file systems.

Example use case:

While working other problems, I download a large torrent in the 
background. The torrent writes to its own, separate file system called 
/foo. If /foo is mounted on its own RAID 10 or 1-LVM array, will that 
help or hinder overall system responsiveness?

It would seem a "no brainer" that giving each major filesystem its own 
array would allow for better threading and responsiveness, but I'm 
picking up hints in various piece of documentation that the performance 
can be counter-intuitive. I've even considered the possibility of giving 
/var and /usr separate RAID arrays (data vs. executables).

If an expert could chime in, I'd appreciate it a great deal.


-- 
Moshe Yudkowsky * moshe@pobox.com * www.pobox.com/~moshe
  "There are more ways to skin a cat than nuking it from orbit
     -- but it's the only way to be sure."
     				-- Eliezer Yudkowsky