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    SubjectRe: [RFC PATCH 35/35] Add Xen virtual block device driver.
    Alan Cox wrote:
    > On Mer, 2006-03-22 at 18:09 -0500, Jeff Garzik wrote:
    >> An IBM hypervisor on ppc64 communicates uses SCSI RPC messages. I think
    >> this would be quite nice for Xen, because SCSI (a) is a message-based
    >> model, and (b) implementing block using SCSI has a very high Just
    >> Works(tm) value which cannot be ignored. And perhaps (c) SCSI target
    >> code already exists, so implementing the server side doesn't require
    >> starting from scratch, but rather simply connecting the Legos.
    >
    > A pure SCSI abstraction doesn't allow for shared head scheduling which
    > you will need to scale Xen sanely on typical PC boxes.

    Not true at all. If you can do it with a block device, you can do it
    with a SCSI block device.

    In fact, SCSI should make a few things easier, because the notion of
    host+bus topology is already present, and notion of messaging is already
    present, so you don't have to recreate that in a Xen block device
    infrastructure.


    > SCSI emulations
    > are also always full of bits people got wrong, often critical bits like
    > tagged queues and error sequences - things that break your journalled
    > file system.

    This I'll grant you.

    Jeff



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