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    SubjectRe: Drunken mumblings [OFFTROPIC]
    On Wed, 30 Dec 1998, John R. Lenton wrote:
    [SNIPPED]
    >
    > Can a SCSI cable be used to connect computers? I mean, can a
    > computer be daisychained with say a hard disk? In theory at
    > least? Using a special card?

    A SCSI disk (or any SCSI device) can be "shared" by other masters on
    the SCSI bus. The master only has to win arbitration (it's done by the
    chip), and it can access any device that wants to be accessed. I use
    "want" here in a particular way. A SCSI Host controller, and its driver
    software, generally doesn't "want" some other SCSI master telling it what
    to do.

    Therefore, communications with a device such as a disk, by two separate
    computers, is not a problem. You can, in fact, use a common hard disk
    even to boot two different machines. The problems come if one or more of
    the machines tries to write to the disk.

    Machine 1 creates a file called /tmp/xxx
    Machine 2 creates a file called /tmp/xxx

    The physical location chosen for the data and the directory will depend
    upon the state of the machine's internal buffering at the time the file
    was created, the OS Version, and when the directory was actually written.

    So you have two files, in two places on the physical media, that have
    the same name and probably different data. This is called file-system
    corruption. After you create/delete a few files for a time (such as a
    kernel compile), the state of the physical media will be entirely
    different than what each kernel "thinks" it is. Recovery from such a
    situation is best done with mke2fs (after disconnecting one of the
    machines).

    If one modified some SYSV configuration files so that log files in
    /var/log and/or /var/adm, went to a separate (private) disk, it is
    possible to have two (or more) Linux machines share the same root
    partition on a shared SCSI hard disk. One only has to make sure
    that this partition is mounted read/only.

    A NFS mounted root partition works differently. You can read/write
    at will because the server performs those functions on behalf of
    each client. The server, therefore, always "knows" and maintains the
    state of its file-systems. Its hard disk is "logically" shared, not
    "physically" shared.

    Cheers,
    Dick Johnson
    ***** FILE SYSTEM WAS MODIFIED *****
    Penguin : Linux version 2.1.131 on an i686 machine (400.59 BogoMips).
    Warning : It's hard to remain at the trailing edge of technology.


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