[Info-vax] Shadow Volume Copy

[JF Mezei]

Ramon Jimenez wrote:

> I tried it but when I could not find the way to tell VMS which shadow
> set member must it consider as master, so the existing one always
> overwrite the copied.


all fully participating disks are equal. But a disk that is the target
of a copy/merge copy isn't a "valid" member yet.

When you mount a DSAx: device, the syste looks at all the hard disks you
supplied as members to find which one(s) are valid and have the most
recent time stamp and that/those ones become the master(s). Any other
disks which don't have the same time stamp/valid flag are the target of
a copy/merge operation.

So, when you reboot, you want to wait until the disk(s) you know are
valid are available before mounting the DSAxL device to ensure that the
valid data is used as master/source or any subsequent copy operation.

For instance, if the first node to boot is one which was offline for a
while (so its disk is not up to date), if you mount the DSAx: device
with that disk, it will become the new master, and when you add more
devices, those will get zapped and overwritten with the datd from the
first node. (in essence overwriting the most recent data with older
stale data)

What I had done was to check to see if all physical devices in a
shadowset were available and if not, wait a couple of minutes in the
boot sequence and then if they are still not all available, prompt the
operator for confirmation to have the shadowset created from the
available disks. (and that required them to call me to explain what had
happened so that a human could decide which disks had valid data.