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VIOC Compatibility Mode, should I turn it off?

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Jeremy Begg
Trusted Contributor

VIOC Compatibility Mode, should I turn it off?



All our systems consist of clusters containing Alpha and Integrity nodes running VMS 8.3 & 8.4, respectively.  Some run just V8.4.


The disks do not have any special caching set up; whatever VMS defaults to is what they have.

Consequently the XFC cache looks something like this:


$ show memory/cache
              System Memory Resources on  2-OCT-2012 14:55:56.63

Extended File Cache  (Time of last reset:  5-SEP-2012 09:13:30.09)
 Allocated (GBytes)             15.57    Maximum size (GBytes)            15.99
 Free (GBytes)                   0.25    Minimum size (GBytes)             0.00
 In use (GBytes)                15.32    Percentage Read I/Os                99%
 Read hit rate                     98%   Write hit rate                       0%
 Read I/O count            4179894929    Write I/O count               31061933
 Read hit count            4099161149    Write hit count                      0
 Reads bypassing cache           5819    Writes bypassing cache        21744798
 Files cached open                849    Files cached closed              63640
 Vols in Full XFC mode              0    Vols in VIOC Compatible mode         9
 Vols in No Caching mode            0    Vols in Perm. No Caching mode        0


You'll notice all volumes are in "VIOC Compatibility Mode".


Is there any performance improvement to be had by forcing these disks to mount in "Full XFC mode" instead?  Any potential problems?



Jeremy Begg


Steven Schweda
Honored Contributor

Re: VIOC Compatibility Mode, should I turn it off?

Hein van den Heuvel
Honored Contributor

Re: VIOC Compatibility Mode, should I turn it off?

Steve, why always the levels of indirections even for simple stuff. Just answer the question already?


"Have you checked the VMS FAQ?"

So you did, and found it right? Why not add just a pointer and/or the result?

Google for : openvms faq "VIOC Compatible Mode" leads nowhere in a hurry.

Google for "openvms faq" first leads to http://www.openvms.compaq.com/faq/openvms_faq.html which is garbage.

It does point to http://www.hoffmanlabs.com/vmsfaq/ which is useful 



in the end you wrote "VIOC Compatibility Mode".

Well that's what I picked up for google string and it made for a miserable hunt.

"VIOC Compatible mode" is easy hunting.






9.10  If my disks are shown as VIOC Compatible, am I using XFC?

                   Yes, you are using XFC caching.

                   Disks that are using XFC caching use communication and
                   coordination protocols that are compatible with the
                   older VIOC caching implementation. With the initial
                   implementation of XFC on OpenVMS, you can use the
                   command SHOW MEMORY/CACHE to see no disks reported
                   in full XFC mode; all disks shown will be listed in
                   "VIOC Compatable Mode".

                   If you have the OpenVMS system parameter VCC_FLAGS set
                   to 2 and are using OpenVMS Alpha V7.3-1 or later, or
                   are using OpenVMS Alpha V7.3 with the VMS73_XFC V2.0
                   ECO kit or later or with the UPDATE kits, you are using

                   Another confusion: the XFC product version is and
                   remains V1.0 in all released configurations, please do
                   not confuse the internal XFC product version (displayed
                   by various commands) with the version number associated
                   with the various ECO kit(s). XFC V1.0 does not permit
                   volumes to enter full XFC caching, as displayed by the
                   "Vols in Full XFC mode" portion of the DCL command SHOW
                   MEMORY/CACHE output.


Jeremy Begg
Trusted Contributor

Re: VIOC Compatibility Mode, should I turn it off?

Thanks Hein.


The OpenVMS FAQ doesn't really answer my question, but pointing out that the text was "VIOC Compatible" rather than "VIOC Compatibility" did help.  Google search brings up the VMS 7.3 OpenVMS System Manager's Manual so I ignored that and found the relevant section in the VMS 8.4 manual.


Here are that manual's comments about the output of the SHOW MEMORY/CACHE command, which I found rather enlightening ...


Vols in Full XFC mode

Reserved for future use by HP. Should be 0.


Vols in VIOC Compatiblode mode

The number of volumes being cached by XFC that are using the VCC caching protocol. As of OpenVMS Version 7.3, XFC uses only VCC caching protocol.


In other words these parts of the SHOW MEMORY/CACHE display are essentially misleading as there is no such thing as "full XFC mode".  (Not as far as customers are concerned, anyway!  Yes I suppose it exists in the system and can be enabled if you know how, but the number of XFC-related patches that have come out over the years would indicate that such a step is not to be taken lightly.)



Jeremy Begg