topic Setting the Shadow set master in Operating System - OpenVMS

Setting the Shadow set master

A.W.R — Thu, 10 May 2007 03:48:52 GMT

Hi,

In a shadow set,is there anyway to set the shadow set master?

We have 2 systems clustered across a 10km link. The disks are shadowed between the sites using Volume shadowing. The primary site does 90% of the processing. The disks show that the shadow masters are on the other site. Can anyone clarify if this will negatively affect performance, and if there is a way to transfer the master ownership to the primary site disks?

Thanks
Andrew

Re: Setting the Shadow set master

Karl Rohwedder — Thu, 10 May 2007 04:09:35 GMT

Andy,

you can use the SET SHADOW command to specify SITE and READ_COST info, so that reads will use the 'local' member. Use SHOW SHADOW to check.

I know of no way to change the master member, except dismounting a member device.

regards Kalle

Re: Setting the Shadow set master

Steve-Thompson — Thu, 10 May 2007 05:25:30 GMT

Hi Andrew

As Kalle states you have some control over read operations.
Shadowing considers write operations as complete only when all members have completed the write operation, so I would hazard it wont make much difference where the master is.

The goal of shadowing is availablity not performance.

Regards
Steven

Re: Setting the Shadow set master

Volker Halle — Thu, 10 May 2007 05:35:13 GMT

Andy,

shadowing is clever enough to automatically load-balance the IO load for the available members, using the device queue length. This does not depend on shadow mastership. Later versions allow management intervention via the READ_COST attribute (especially for Fibre SCSI disks), which allows you to give a hint, which disks are 'local' vs. 'remote'.

You can use SHOW DEV/FULL dev and have a look at the operation count for both members. Depending on the read/write ratio you should see a (much) higher operation count on the local member.

Volker.

Re: Setting the Shadow set master

Robert Brooks_1 — Thu, 10 May 2007 08:49:05 GMT

The previous responses are all correct.

Please see the SYSGEN param SHADOW_SITE_ID in order to associate devices with a specific "site". Using the READ_COST mechanism will work as well, however.

Also, please consider using HBMM if you are not already doing so.

-- Rob

Re: Setting the Shadow set master

Petr Spisek — Fri, 11 May 2007 07:58:10 GMT

If you mean Master Member (checked by command SDA> show dev dsa100) so this is managed by shadowserver (last mount counter - or something similar).
For SAN configuration is useful SET DEVICE/SITE=... for reading from local member.
Write is managed by shadowserver.
Petr