Maurice Hilarius wrote:

A friend writes:

On a recent machine set up with Raid5.
On a AMD Phenom II X4 810, and 4GB ram.
4 Seagate 7200.12  SATA 1TB drives,

I'm getting some rather impressive numbers for sequential read
(300MB/s+) and write (170MB/s+) but the random read is proving to be
absolutely atrocious.
iostat says its going at about 0.5MB/s,

The key thing about random i/o is the block size.  With, say, 512bytes
blocks and single thread you will see less than 0.5Mb/sec.  With 64kbytes
blocksize it will be much better.

To diagnose: first try the same test on bare disk without raid layer.
Next try to vary block size and number of concurrent threads doing I/O.
There's no tweaks needed really.

/mjt