[Info-vax] New filesystem mentioned

[Mark Berryman]

On 5/14/19 9:36 AM, Dave Froble wrote:
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 > .
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> So, I don't understand the comment about DLM calls in ZFS?  Locking is 
> not part of a file system, locking is used in the tools that access a 
> file system.  Things such as a database, or RMS, or other such.

That would not be correct.  At a minimum, access to the metadata of the 
file system must be coordinated.  Using the allocation bitmap as one 
example, you have the following:

For a non-shared filesystem you have a couple of choices:
1. You single-thread all access to the bitmap so there is never more 
than one writer.
2. You take out a lock in local memory to coordinate updating the bitmap.

For a share filed system, somewhat more work is involved.

Given: node A and node B in a cluster.  Both node A and node B have 
cached the current copy of the allocation bitmap.

Node A needs to make a change.  It requests an exclusive DLM lock to 
change the bitmap.  DLM will notify all other nodes that node A now has 
an exclusive lock and they must now invalidate their cached copy of the 
bitmap.  Node A updates the bitmap, and probably schedules a disk write 
thereof.

What node A now does with the lock depends on the sharing strategy.  If 
metadata is shared by flushing to disk, node A can't convert the 
exclusive lock back to shared mode until after the disk write completes. 
  That will allow another node's request for an exclusive lock to 
complete and they will then read the updated bitmap back into memory.

Or, perhaps there is an algorithm to inform the other nodes of what 
change was made.  In that case, node A can't release the exclusive lock 
until all other nodes inform node A they have made the necessary 
updates.  This would alleviate the need to flush through disk but would 
require some sort of algorithm to decide by whom and when write-backs 
are scheduled.

Now another node needs to modify the bitmap.  That node needs to request 
an exclusive lock and can't proceed until it gets it.  Loop.

This is just one piece of metadata to which access must be coordinated, 
and coordinated in a manner that just doesn't exist on a non-shared 
filesystem.

So, yes, there would be a number of changes required to take a 
filesystem from just about any non-VMS system and make it work on VMS.

Mark Berryman