[Tfug] Crappy USB LVM/Software RAID10 performance?

[Bowie J. Poag]

Mmmkay.... Here's an interesting conundrum..

I just bought two 750GB external USB drives to replace the internal 
2x120GB hardware RAID1 setup I have here at the house.  Each drive is 
plugged into a seperate USB hub on my box. I set up the drives in a 
RAID10 relationship to make them as fast as possible. From there, I 
created a volume group on the resulting device, built an logical volume 
within it spanning the whole drive, formatted it as ext3 and mounted it. 
No problems anywhere along the line, no hiccups, nothing.

Now here's the problem..

On writes, i'm seeing only 13-15MB/sec, which sucks.
On reads, i'm seeing 19-20MB/sec, which also sucks, but sucks a little less.

USB 2.0 is spec'ed @ 480Mb/sec max, which means the pipe should be able 
to handle upwards of ~60MB/sec under ideal circumstances. These are 
probably SATA drives i'm working with, ones easilly capable of 
35-40MB/sec regardless of method. Even if they were IDE drives, i'd 
expect better performance, upwards of 25-30MB/sec.

So, here's the puzzle..

dmesg shows that both drives are recognized as USB 2.0 devices, so it 
isn't a legacy 1.x->2.0 USB problem..
I have no other traffic moving over the pipe besides keyboard and mouse, 
so I/O shouldn't be a problem.
This box isn't doing anything else.

So where's the crappy performance stemming from?

Cheers,
Bowie