State databases for shared storage

Question

When adding a disk to a disk-set (Solaris Volume Manager), it automatically adds a state DB to slice 7 and then of course to the next disk added.
Sun Cluster recommends 3 state databases on each disk of an internal. Is there a recommendation for the shared disks. I know one is added automatically, should it have more?

Answer 1

No, you don't need to an any normally. SVM will automatically balance the replicas across the disks added to the set. The only time you might want to mess with this default allocation is if you want to create a preferenced site. In the case where you have two sites and an even number of disk arrays located across these sites, i.e. nodeA & arrayA in siteA, nodeB & arrayB in siteB, you might want to make site A preferred.
SVM works on the principle of requiring a majority of replicas to allow a diskset to be imported read/write. Mediators help if you have a system failure then at some point later an array failure by allowing the internally held mediator to go 'golden'. However, if you have an instantaneous site failure, then the array and the server are loss simultaneously. The mediator on the remaining site does not go golden. Thus any diskset previously held by the failed site cannot be brought in read-write because there are only 50% of the replicas available (on say arrayB). You can manually delete some 'dead' replicas and get the set imported, but that's not very highly available. Alternatively, you an add an additional replica to siteB's arrayat configuration time and this would give it the majority it needed. However, if site B failed, you are in the same situation! The best option is to have a 3rd site with some of the storage on.
Why does it work this way? Because if you were in the middle of a change to the configuration at the point of failure you might get the wrong info about the disk arrangement on a set switchover and possibly corrupt data. Data corruption is highly undesirable, so SVM protects you by forcing a manual intervention to resolve the issue.
Don't know if that helps?
Regards,
Tim
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