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DateFri, 24 Mar 2000 11:48:50 +0100
FromHelge Hafting <>
SubjectRe: Fake SCSI devices
>>Machines connected in this fashion may also share scsi devices.  The
>>read-only cases are trivial, you must take grat care with anything writeable.

>Writeablity would be very desirable! How much does Digital sell their SCSI
>hubs for?

You can share writeable stuff. The problem is software, not equipment.
Basically, only one of the two (or more) machines can use a writeable
partition at once.

So several machines may mount the same partition read-only (or the same
cdrom)
Only one can mount a partition for writing. Other partitions may still
be
read/written by other machines. The machines can take turns writing to
a partition,
but one must unmount before another gets it.

This because each machine have a disk cache, and can't know when the
other is writing to the disk. Cache contents will be wrong after such a
write.
You probably can't snoop a scsi bus, and even that would be open to
races.

If you want both to mount the same partition for writing (or one writing
and another simultaneous read-only) please write a new filesystem where
each machine
ask all the others for permission (and invalidate their cached blocks)
before each write.

Helge Hafting

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