Maximum shadow copies

[Mike Smith_33]

I have a new cluster I have been setting up and I have noticed something strange. I set shadow_max_copy to 4. I have noticed that the system upon startup begins shadowing all 7 drives at once. What am I missing here?

Cluster is 3 node, DS15s clustered over the network. 2 nodes have msa30 shelves and we then host based shadow all the drives between them. They each have a private network between them weighted (GB) for disk and cluster traffic. The third node is a voting node and talks to the other two over the public network.

[Dave Unverhau_1]

Mike,

I'm a VMS newbie, so please excuse me if I'm missing the point...

According to the "Volume Shadowing for OpenVMS" manual, the shadow_max_copy system parameter "limits the number of concurrent merge or copy operations on a given node".

This may mean that it limits how many shadow *sets* are able to be merged or copied simultaneously. Do each of your seven disks on this node constitute a separate shadow set? If I understand the purpose of the parameter correctly, it will only allow copies of up to four shadow sets to occur simultaneously. Depending on how your disks are set up in shadow sets, might this involve read and write operations to all seven of the disks?

I hope this is of some help...let me know if I'm off base...I'm learning, too (I hope!).

Best Regards,

Dave

[Robert Gezelter]

Mike,

I must concur with Dave. shadow_max_copy is PER NODE, not PER CLUSTER.

Are you sure that the shadow copies are all being performed by the same node, or is the status display showing the overall state of the volumes. Perhaps you can post the a copy of the display that you are looking at (suitably sanitized, or course) so that we can all look at the same thing.

- Bob Gezelter, http://www.rlgsc.com

[John Gillings · Accepted solution]

SHOW SHADOW/COPY should show you copies in progress, include which node for which shadowset(s). If you have 3 nodes, each with SHADOW_MAX_COPY set to 4 then you can perform 12 copies simultaneously.

One school of thought says the CURRENT value of SHADOW_MAX_COPY should be 0 on all nodes, so copies do not start automatically on reboot. Once the systems are up, and you've verified the existance of all virtual units, you can selectively enable copies on specific nodes by setting a non-zero value. This ensures you don't copy in the wrong direction.

There are all kinds of controls on shadow copies and merges these days (V7.3-2 and up) with preferred hosts, priorities, delays, policies etc... see the shadowing docs for all the details.

[Mike Smith_33]

Maybe I have misunderstood what the setting does. I have the following 7 shadowsets mounted across the two nodes with msa30 shelves.

DSA1: Mounted 0 INGRES 67429323 70 2
DSA2: Mounted 0 CHKPT 70584381 5 2
DSA10: Mounted 0 DATA0 70614192 1 2
DSA11: Mounted 0 DATA1 70674771 1 2
DSA12: Mounted 0 DATA2 63602826 1 2
DSA13: Mounted 0 DATA3 70503444 5 3
DSA14: Mounted 0 DATA4 70665615 1 2

I thought by setting the param to 4 on all nodes, it would allow four of the shadowsets maximum. If it is per node then it makes sense that all seven could go. One node has four while the other has four.


Thanks guys.

[Robert Brooks_1]

Hi Mike,

As previous responders have noted, I believe you misunderstand the purpose of SHADOW_MAX_COPY. That parameter has absolutely nothing to do with the number of shadow sets that can be mounted on a system. THAT parameter is called SHADOW_MAX_UNIT. The default is 500 on Alpha and I64; the default is 100 for VAX.

Once again, the purpose of SHADOW_MAX_COPY is to control the number of shadow sets per system that undergo either a merge or a copy. The default for this parameter is 4, meaning that up to 4 shadow sets can undergo a merge or copy on this system. Please note that

If you are unclear about the concept of merges or copies, please let us know and we'll attempt to explain them.


-- Rob (VMS Engineering)

[Mike Smith_33]

This was an RTFM situation. I had not fully understood the node versus cluster setting of the shadow_max_copy parameter.