We are using an IBM3584 and don't use any kind of collocation.  We have found 
the migration process from diskpool to tape collocates data by node and 
therefore already does a decent job of grouping node data together.

The only time you may see better performance with collocation is with a server 
that has alot of little files that change often and those nodes should probably 
be stored on VTL or DISK rather than tape.

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