I share DASD all the time. Chris Little put it well:
> The issue that goes beyond this is that Linux doesn't understand
> shared devices. It can't synchronize writes at the OS level. ...
Right. Last I checked, the kernel cached blocks of data
for block devices (all disk, shared or not!) making shared disk
very dangerous if ANY of the participants had a R/W mount of it.
Vital points if you wish to try sharing DASD:
all Linux images using the disk must mount it R/O
virtual machine should define the disk R/O
MDC off for the shared disk (but see Rob's comment)
I despise the boot parm syntax for (ro)
and maintain that the driver should detect "write inhibit".
But some have suggested that as well.
Rob van der Heij pointed out that it may be a performance hit
to switch off MDC. Personally, I've had enough CMS volumes
(not even Linux stuff) fouled-up by MDC that I turn it off
if I know one side has R/W. But again, to share disk among Linux,
DON'T USE R/W, and you can leave MDC enabled.
Obviously you will need to have R/W access at some time.
Do such work in a maintenance period and then re-IPL all systems
which might have cached anything.
NFS is really a silly way to share disk if you have z/VM.
The protection it offers is not worth the cost.
There are always exceptions.
-- R;
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