[Info-vax] New filesystem mentioned

[Dave Froble]

On 5/14/2019 2:28 PM, Simon Clubley wrote:
> 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.
>

This is scary, Simon and I thinking alike ....

:-)

-- 
David Froble                       Tel: 724-529-0450
Dave Froble Enterprises, Inc.      E-Mail: davef at tsoft-inc.com
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