Mount same ext3 filesystem in 2 places... at the sametime

[Anton Verevkin]

On Tue 29/09/09 23:08 , James Knott <james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org> wrote:
> I haven't worked with SCSI or fibre channel gear, but many years
> ago, I
> used to work on mini computers, which could share drives.  On those
> systems, there was an interlock in the disk interface that prevented
> both computers from writing at the same time.

I have recently tried to connect two computers with SCSI - just for a personal experiment.
The articles that you can find in the Internet all say that this is theoretically possible, however, 
usually turns into many compatibility issues on practice. I used two exactly same Adaptec 
controllers with the same firmware version. I disabled termination on controllers, changed 
SCSI ID from the default on one of them and connected them and a shared hard disk with 
a SCSI cable which I terminated on both sides with hardware terminators. 

And surprisingly it worked without any problem. One computer had Gentoo, another one had 
FreeBSD. I created a vfat partition on the shared disk and both computers were able to read/write to it.

But this is exactly the case when computers do not recognize that they are not alone using 
this partition. Though SCSI bus (and Fibre Channel as its successor) do handle the situation 
of simultaneous access to disk and have disk locking procedures, operating systems were 
not aware of changes made by another computer and I had to clear disk cache to see the 
changes made by another system. And of course simultaneous write could damage the data. 

I guess there might be some tools to teach the operating system to work in a team with other 
computers, and most probably this will turn to something like ZFS. But anyway we should 
always be very careful with these matters.

Best regards, 
Anton Verevkin 

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