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    SubjectRe: [ANNOUNCE] [PATCHES] Advanced TCA Hotswap Support in Linux Kernel

    Greg KH wrote:

    >On Tue, Oct 15, 2002 at 01:46:34PM -0700, Steven Dake wrote:
    >>The data/telecoms I've talked to require disk hotswap times of less then
    >>20 msec from notification of hotwap to blue led (a light used to
    >>indicate the device can be removed). They would like 10 msec if it
    >>could be done. This is because of how long it takes on a surprise
    >>extraction for the hardware to send the signal vs the user to disconnect
    >>the hardware.
    >But what starts the "notification of hotswap"? Is this driven by the
    >user somehow, or is it a hardware event that happens out of the blue?
    In the case of Advanced TCA, an IPMI message is sent to the CPU blade
    indicating the hotswap button is pressed on the front panel of a disk
    blade. The hotswap manager software unmaps the GA address, removes the
    device from the linux kernel via the scsi-hotswap-main stuff, and sends
    another IPMI message to the disk node telling it to light its "blue
    led". The user removes the disk. Insertion is easier.

    In this case, the hotswap button on the front panel is used to indicate
    a hotswap event. There is talk of making the removal of the board
    indicate a hotswap event (surprise extraction) because the technicians
    don't wait for the blue led to remove the boards occasionally and the
    system should be able to handle this use case.

    >>For legacy systems such as SAFTE hotswap, polling through sg at 10 msec
    >>intervals would be extremely painful because of all the context
    >>switches. A timer scheduled every 10 msec to send out a SCSI message
    >>and handle a response if there is a hotswap event is a much better course.
    >What generates the hotswap event?
    In the case of SAFTE, a SCSI processor (ASIC) is polled by some polling
    interval about the state of the SAFTE (SCSI) backplane. When the state
    changes, software generates a hotswap event and removes the device.



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