[AFS3-std] Proposal: Extension of RXAFS_SetLock to support LockUpgrade and LockDowngrade

[Jeffrey Altman]

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Jeffrey Hutzelman wrote:
> 
>> In order to avoid the need to issue try the new operations and
>> fallback to the old release and lock approach, a Capability flag
>> VICED_CAPABILITY_LOCK_UPGRADES will be created using a number assigned
>> by registrar@grand.central.org.
> 
> Why is a capability necessary in this case?

For the reason I explained above.  Capability bits are cheap.  Try and
fallback is expensive.  Its not safe to cache the knowledge gained by
trying and failing.  Capabilities are refreshed every time the server
is probed if GetCapabilities is used instead of GetTime.  Therefore,
the client always knows whether or not the functionality is supported.

In case it is not clear to you, if the atomic LockUpgrade operation is
not supported then the clients must fallback to a combination of
LockRelease / LockWrite and cross its fingers that the LockWrite can be
obtained.  If not, it must LockRead.  In both cases it has to verify
that the DataVersion has not changed.  If the DataVersion has changed
the cache manager must return an error to the calling application and
prevent all further use of the file.

Performance of AFS3 is already bad enough.  I am not going to make it
worse by insisting that clients try and fall back, thereby expending an
extra round trip time when a capability bit could be used that costs
zero network time.

Jeffrey Altman




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