[Info-vax] New filesystem mentioned

[Dave Froble]

On 5/14/2019 2:05 PM, Michael Moroney wrote:
> Dave Froble <davef at tsoft-inc.com> writes:
>
>> On 5/14/2019 9:56 AM, Bob Koehler wrote:
>>> In article <qbco24$r22$2 at dont-email.me>, Dave Froble <davef at tsoft-inc.com> writes:
>>>>
>>>> Not that I'm an expert on VMS clusters, but it has been my impression
>>>> that the OS did all the work.  But what do I know?
>>>
>>>      The file system is very much part of the OS.  If it's going to work
>>>      in VMSclusters, it must support VMSclusters.
>>>
>
>> Why would any filesystem not support VMS clusters?
>
> One word answer: Caching.

Oh, yeah, one of my favorite boogymen.

> Any reasonably advanced file system will have a disk cache, that is memory
> containing the contents of recently/frequently accessed data.  On a system with
> many processes interested in the same records, the file system can easily share its
> cached content to all interested processes.  Writing is a little more complex,
> the file system will have to lock out other requests for the length of time it
> takes for an atomic write to be performed and the cache updated.

Before the next paragraph, I've got to ask, is caching really part of 
the file system?  Yes, it's got to know what to do with the cached data, 
but is it part of the file system, or something that uses it?

> Now enter VMS clusters. Now there is a situation where some nodes may have the data
> in cache and others not.  They had better have the same content in cache and on
> disk!  Additionally, the cached data one node has is NOT visible from another node.
> Only the on-disk data is.  And now if node A wishes to write to a file, it must
> inform all nodes (via the DLM) that it wishes to do so, and all nodes will have to
> dump (invalidate) any affected cached data before it can grant permission to do the
> write.  Once the write is done, it must be certain the data made it all the way to
> the disk before allowing the other node(s) to read the data back in (remember cache
> was invalidated). What file systems have this level of synchronization, other than
> ODS-x, even if you assume some sort of 'black box' synchronization (like DLM)
> without worrying about its details?

I do like your reference to the DLM as a black box.  It just works. 
Don't worry about how.

> I've been heavily involved with disk sh*d*w*ng in the past.  One of the reasons why
> it is so freaking complex is that it has to do similar messy coordination when
> individual nodes can come and go as well as individual member drives can come and
> go, all asychronously.  That is sh*d*w*ng cannot say to a node "wait, you can't
> crash until I do this and that!".
>
>> A file system is the structure on disk, and the code to access and
>> support it.  If any node has access to the device, then it has access.
>> What more is needed?
>
> oh if only it were that simple.
>

Well, I'm a simple guy.

I tend to look at things from an architecture perspective.  The dirty 
details can be much worse.  And too many times are.

-- 
David Froble                       Tel: 724-529-0450
Dave Froble Enterprises, Inc.      E-Mail: davef at tsoft-inc.com
DFE Ultralights, Inc.
170 Grimplin Road
Vanderbilt, PA  15486