External USB2 HDD affects speed hda

[Rene Herman <rene.herman@keyaccess.nl>]

Hi Bartlomiej.

My Maxtor 6Y120P0 on AMD756 (UDMA66) normally gives me 50 MB/s according 
to hdparm -t:

===
# hdparm -t /dev/hda

/dev/hda:
  Timing buffered disk reads:  152 MB in  3.01 seconds =  50.57 MB/sec
===

However, the second I switch on my external USB2 drive (Western Digital 
Essential 160G, connected via a PCI card USB2 controller, on a private IRQ):

===
usb 1-3: new high speed USB device using ehci_hcd and address 3
scsi0 : SCSI emulation for USB Mass Storage devices
usb-storage: device found at 3
usb-storage: waiting for device to settle before scanning
   Vendor: WD        Model: 1600BB External   Rev: 0412
   Type:   Direct-Access                      ANSI SCSI revision: 00
SCSI device sda: 312581808 512-byte hdwr sectors (160042 MB)
sda: assuming drive cache: write through
SCSI device sda: 312581808 512-byte hdwr sectors (160042 MB)
sda: assuming drive cache: write through
  sda: sda1 sda2
Attached scsi disk sda at scsi0, channel 0, id 0, lun 0
usb-storage: device scan complete
===

the hdparm -t result drops down to 42MB/s:

===
# hdparm -t /dev/hda

/dev/hda:
  Timing buffered disk reads:  130 MB in  3.04 seconds =  42.77 MB/sec
===

Switching the USB2 HDD off again does not work to bring back the 50 MB/s:

===
# eject sda
# hdparm -t /dev/hda

/dev/hda:
  Timing buffered disk reads:  128 MB in  3.01 seconds =  42.57 MB/sec

[ push button ]

usb 1-3: USB disconnect, address 3

# hdparm -t /dev/hda

/dev/hda:
  Timing buffered disk reads:  130 MB in  3.04 seconds =  42.73 MB/sec
===

After a reboot, it's 50 MB/s again. Any idea what this is?

The USB HDD is not firing interrupts or anything. It just sits idle. 
Fully repeatable on 2.6.11.11 and 2.6.12-rc5.

Rene.