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SubjectRe: Fake SCSI devices
Helge Hafting wrote:

> >>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.
>

Simple trick is to have the filesystem ask the "hosting" (i.e. the one who opens
it first and has write access) to do the write for you transparently. Or, more
simply, if you find that the partition is open, simply make the mount into an NFS
mount instead. Voila, you have (in some, very small way) something like
TruClusters. Of course, the technology they use is just a twitch more complicated
than that, but it's the general idea.

- Pete

>
> 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.


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