[Info-vax] Current VMS engineering quality, was: Re: What's VMS up to these

[glen herrmannsfeldt]

Johnny Billquist <bqt at softjar.se> wrote:

(snip regarding NFS mounts of disks that go away.)

>> I haven't done it for a while, but most often you can reboot without
>> accessing the specific disk.

> If the root fs is over NFS, you obviously can't...
> But yes, if you can totally avoid ever accessing the NFS file system, 
> then you can (hopefully) carry on.

It is possible, but not so common, to have an NFS root and also
export disks to other NFS clients. 

(snip, I also wrote)

>> In general, Unix doesn't do much file locking, and, yes the NFS
>> implementation is especially troublesome.

> Indeed. But it is worse. For example, if a file is opened for execution, 
> Unix prevents you from writing to that file. 

Not on systems that I used. Which ones do that?

> This semantic is also lost if the file is accessed over NFS, 
> meaning you can write to a binary that is being executed, 
> causing the memory of that executable to randomly 
> corrupted at execution time, depending on when/if page in 
> from the binary happens.

That does happen.

(snip)

>> Unix convention is that if you delete an open file, it stays around
>> until closed, though the name is removed from the directory.
>> It isn't possible to do that with a stateless NFS server.

> Right. So when NFS gets involved, it becomes a bunch of hacks, where 
> data corruption can happen (that Unix otherwise will not allow), and 
> weird solutions are used to get around the different semantics in NFS.

Well, first only do hard mounts. Soft or interruptable mounts
definitely can cause those problems. But hard mounts will wait
forever for the server to come back. (Once I had to shut down a 
server for a weekend. The clients waited for it to come back.)

(snip)
>> I am not sure exactly how much has to change. If something relating
>> to the specific disk changes, then yes.

> No. It's basically if anything appears different in the hardware 
> configuration. Enumeration of devices are a part of NFS, and if that 
> enumeration is suspected to have changed, then all old file handles 
> are invalidated. Note that it does not necessarily have to have 
> changed, it's just the suspicion that it has changed that is 
> enough to trigger this.

It is likely different on different systems. I am pretty sure
that I did some changes where they didn't go stale, and others
where they did.

(snip)

-- glen