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    SubjectExternal USB2 HDD affects speed hda
    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.
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