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    SubjectRe: XFS and journalling filesystems


    On Thu, 27 May 1999, Theodore Y. Ts'o wrote:

    > Date: Mon, 24 May 1999 11:24:35 -0600
    > From: Larry McVoy <lm@bitmover.com>
    >
    > : (And there are certain features of XFS, such as the features that allow
    > : Irix to tell the disk controller to send disk blocks directly to the
    > : ethernet controller, which then slaps on the TCP header and calclates
    > : the TCP checksum without the disk data ever hitting memory
    >
    > This is not quite right, in fact, it is a little unfair to IRIX. There is
    > no interaction between the file system and/or the block device system
    > and the networking stack. The way it works is this (I know this code
    > extremely well since I'm the guy that originally made NFS use both the
    > networking and the file system to go at 94MByte/sec - Ethan Solomita is
    > the guy who made it go at 640MByte/second over Super HIPPI):
    >
    > I stand corrected. I had heard about the zero copy algorithm you
    > described, but then when I later heard that Irix had bypassed even the
    > DMA in and out of memory, I was appropriately horrified. I'm glad to
    > hear it wasn't true, and that you had no part in implementing or
    > designing such a horrible hack (i.e., direct device-to-device I/O). :-)

    You can design I/O Buses on which the CPU uses the same bus protocol as
    normal I/O devices and that allows any device to access the whole memory
    and memory mapped IO space. PCI is obviously a good example of that. (Note
    that it seems that, when only CPU-to-brain-wired-O/S-guys are involved in
    the design of an interface with a PCI sub-system, they used to allow the
    CPU some features regarding BUS accesses that normal I/O devices haven't.
    This is an hack too, in my opinion).

    It is possible by design on those Buses for any device acting as a master
    to directly talk to another device acting as a target and it is this way
    I/Os involving the CPU are _actually_ working. In order for direct I/O to
    I/O not looking like a hack, we probably need to have sophisticated I/O
    controllers that can host some logic that is currently part of the O/S.
    This let me think that it is possible, at least on paper, to implement
    such a system that is absolutely not a hack.

    Gérard.


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