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Setting the Shadow set master

Frequent Advisor

Setting the Shadow set master


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?

Karl Rohwedder
Honored Contributor

Re: Setting the Shadow set master


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
Regular Advisor

Re: Setting the Shadow set master

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.

Volker Halle
Honored Contributor

Re: Setting the Shadow set master


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.

Robert Brooks_1
Honored Contributor

Re: Setting the Shadow set master

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
Petr Spisek
Regular Advisor

Re: Setting the Shadow set master

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.