tuning parameters for parallel access to umass devices?

[Brian Behlendorf]

It looks like copying files over USB to a umass device causes other I/O
attempts to be blocked for an unreasonable (10-20 seconds) amount of time.
Here's the device in question:

da2 at umass-sim0 bus 0 target 0 lun 0
da2: <Maxtor 5000XT v01.00.00 0100> Fixed Direct Access SCSI-0 device
da2: 650KB/s transfers
da2: 239371MB (490232832 512 byte sectors: 64H 32S/T 42763C)

Though I've seen it with other umass devices, like my digital camera and
the Archos jukebox.

While copying a large file (or large number of files), and in parallel
attempting to do a "ls" of a directory or read a file, one waits a
significant amount of time.  Is there a way to tune the disk, mount
options, kernel sysctls, something, to make file I/O over USB less
blocking?

	Brian