Depends on how you define DR - we have shared storage HA in each datacenter (NetApp cluster), and replication between them in case we lose a datacenter (all clients on the MAN hit the same cluster unless we do a DR failover). The latter is what I'm calling DR.

It's what I call HA. DR is what snapshots or backups can help you towards. HA can be used to reduce the likelyhood of needing to use DR measures of course.

"seamless" is an over-statement, I agree. NetApp has synchronous SnapMirror (which is only mostly synchronous...). Worst case, clients may see a filesystem go backwards in time, but to a point-in-time consistent state.

Tell that to my swapfile! Here we use synchronous mirroring for our VM systems storage. Having that go back in time will cause unpredictable problems. Worst case is pretty bad!

It may be that for your purposes you can treat your filesystems the way you do safely - although you'd better not have any in-memory caching of files, obviously - however lots and lots of people cannot.

I believe that we can do seamless replication and failover of NFS/ZFS, except that it is very painful to manage, iSCSI (the only way I know to do mirroring in this context) caused us a lot of pain last time we used it, and the way Oracle treats Solaris and its support has made it largely untenable for us.

Instead we've switched to Linux and DRBD. And if that doesn't get me sympathy I don't know what will.


Julian King
Computer Officer, University of Cambridge, Unix Support
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