[Info-vax] New filesystem mentioned

[Simon Clubley]

On 2019-05-14, Michael Moroney <moroney at world.std.spaamtrap.com> wrote:
> Dave Froble <davef at tsoft-inc.com> writes:
>
>>Why would any filesystem not support VMS clusters?
>
> One word answer: Caching.
>
> 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.
>
> 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?
>

But isn't this a problem in the filesystem drivers instead of in the
filesystem itself ?

I understand what you say above, but I am not seeing anything which is
ODS-2/ODS-5 specific.

This seems more about instances of the filesystem drivers running on
different nodes using the DLM to synchronise caching between themselves
as opposed to anything that's actually stored in the filesystem on-disk
structures themselves.

I don't see anything here that would stop a FAT32 cluster-aware filesystem
driver (for example) from also using the DLM to coordinate the caching.

Simon.

-- 
Simon Clubley, clubley at remove_me.eisner.decus.org-Earth.UFP
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