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Highwater marking on a shadow set

Cor Mom
Advisor

Highwater marking on a shadow set

I have a number of systems running in a real time environment using shadow sets. Now I would like to disable highwater marking on the shadow sets. Will I have to do this on each shadow set member, or just on the shadow set device, DSA1: ? In the later case, will this setting remain after a reboot?

Somewhere in the OpenVMS docs I found a line that said to remount the volume after changing the highwater marking. Is this true?

Thanks.
OpenVMS - Nothing stops it
5 REPLIES
Uwe Zessin
Honored Contributor

Re: Highwater marking on a shadow set

This is a change to the disk volume structure - you have to do this on the shadow virtual unit (DSA1:) - the physical members are not accessibe for this type of operation. Last time I checked, this information is permanently recorded on the disk - you might have to execute the command on any cluster member that has the disk mounted, though.
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Kris Clippeleyr
Honored Contributor

Re: Highwater marking on a shadow set

Hi Cor,

Welcome to this forum. Long time no see.

The SET VOLUME /[NO]HIGHWATER_MARKING works on volumes (Files-11 ODS-2 & 5 disks).
You simply modify the virtual unit (DSAx:), not the individual members. No dismounting & mounting needed. The setting is persistent across reboots (provided it is not changed somewhere in your SYSTARTUP_VMS.COM, when mounting the disks).

Greetz,

Kris (aka Qkcl)
I'm gonna hit the highway like a battering ram on a silver-black phantom bike...
Cor Mom
Advisor

Re: Highwater marking on a shadow set

Uwe and Kris,

Thanks.

Kris,

Indeed, it has been quite some time ago. It noticed you some time ago on this forum, and yes, this was the first time for me.

See you.
OpenVMS - Nothing stops it
John Gillings
Honored Contributor

Re: Highwater marking on a shadow set

Cor,

Highwater marking (HWM) is a volume characteristic, so you set it on the DSA device, not the physical members. It's persistent, so the setting (on or off) will survive remounts and reboots.

However, enabling or HWM on a volume on one node in a cluster DOES NOT propagate the setting to other nodes. It's possible to have the SAME volume with HWM enabled on some nodes and disabled on others simultaneously (hmmm, I wonder if we could twist that into a "feature"? ;-). I'm guessing that's the reason for recommending a remount - so that the VCB gets read from all nodes to make the HWM setting consistent across the cluster.

I'm fairly sure you can safely enable or disable HWM without remounting the volume - just make sure the SET VOLUME command is executed on every node in the cluster (use SYSMAN with SET ENVIRONMENT/CLUSTER).

To confirm if HWM is on or off try this:

$ COPY NL: AFILE.TXT/ALLOCATE=10
$ SET FILE/END AFILE.TXT
$ DUMP/BLOCK=COUNT:2 AFILE.TXT

If HWM is enabled, you'll see two blocks of zeros. If it's disabled, you'll see remnants of a deleted file (typically non-zero). The enabled test assumes there have been files with nonzero contents recently deleted (disk block allocation is LIFO). So, seeing zeros doesn't confirm HWM is enabled (since you might have just happened to get zero blocks allocated), on the other hand, if you see anything non-zero you know for certain that HWM is disabled. If in doubt, create a large file, fill it with anything, delete it, and repeat the above sequence.
A crucible of informative mistakes
Cor Mom
Advisor

Re: Highwater marking on a shadow set

John,

I used your commands to check the highwater settings from all cluster members and I can see old data in AFILE.TXT, so its working.

Thanks a lot.
OpenVMS - Nothing stops it