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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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