Re: Sharing SCSI disks

Richard B. Johnson (
Thu, 20 Mar 1997 18:48:07 -0500 (EST)

On Wed, 19 Mar 1997, Alan Cox wrote:

> > Note that only ONE computer can have ACTUAL PHYSICAL access to the
> > SCSI Disk. The other machines must have virtual access, which means
> > that one machine only does the physical I/O. This can be explained
> > as follows: Suppose you had two computers connected to the same SCSI
> > drive. Suppose one of them went to read or write from the drive and
> > found that the SCSI bus was busy. What would it do? It would reset
> > the bus and retry. The other computer would then find that its
> > I/O was aborted. It would then reset and retry. This would go on
> > "forever" even if it was a read-only device!
> On the contrary SCSI has full multi host/target arbitration - it depends
> on your controller how happy it is but its fine, and sharing an ext2fs
> disk Read only is just perfect
Not true. It depends upon the DRIVER. There isn't a driver that I've used
in Linux (or Sun) that knows what to do when some other host takes over
the bus. Try it. The first error will be the bus-phase error. The rest are
all down hill from there.

We thought it would be a real good idea to share a CD-ROM between two
Linux Boxes. After that fiasco, I tried it on my Suns. Just because
a bus Interface Specification allows any SCSI Host to be a Master, doesn't
mean that an Operating System will allow it!

We have a RAID assembly that strips SCSI drives. The SCSI I/O ports go
to a Board with a '68000 and 330 megabytes of RAM, not to the physical
drives. The 68000 is "in charge" of the drives. It is the Lock Manager.

Dick Johnson
Richard B. Johnson
Project Engineer
Analogic Corporation
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