Moshe Yudkowsky wrote:

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).

One partitionable RAID-10, perhaps, then partition as needed. Read the 
discussion here about performance of LVM and RAID. I personally don't do 
LVM unless I know I will have to have great flexibility of configuration 
and can give up performance to get it. Other report different results, 
so make up your own mind.

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.

-- 
Bill Davidsen [off-list ref]
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark