volume shadowing - disks with different maxfiles/cluster size

[MarkOfAus]

Hi all,
A question:

If volume shadowing 2 disks, each exactly the same physically but each has different maxfiles/cluster size, which does volume shadowing use?

set volume/limit explicitly states it uses init/cluster as its upper limit, but which, the lower of the two?

Thanks,
Mark

[Andy Bustamante · Accepted solution]

If the disks are the same size, your mount command(s) will determine which disk is over written. The maxfiles and clustersize from the source disk will replace values on the target disk.

Later versions of VMS support shadowing with disks of different size. This can be used to incease volume size "on the fly" without an outage, assuming the volume was initialized planning for growth.

Andy

[John Gillings]

Mark,

>exactly the same physically but each has
>different maxfiles/cluster size, which
>does volume shadowing use?

In a shadowset, both disks are identical, so there can be no difference in maxfiles or cluster size.

Ideally you will have done:

$ INIT/SHADOW=(disk1,disk2)/CLUSTER=c/MAX=m/HEADERS=h etc..

prior to forming the shadowset, so the question is moot because everything will already be identical.

However, if you have taken 2 random disks and formed a shadow set:

$ MOUNT/SHADOW=(disk1,disk2) etc...

the shadowset is established with disk1, which is then shadow copied to disk2. All data and attributes of disk2 are overwritten with those of disk1. Since this has the potential to destroy data inadvertently, for safety, I'd recommend any shadowset mount have:

/POLICY=(REQUIRE_MEMBERS,VERIFY_LABEL)

VERIFY_LABEL prevents the wrong disk from being overwritten, and REQUIRE_MEMBERS requires that all devices be available, which prevents the shadow copy from being done in the wrong direction.

[MarkOfAus]

Thanks.