There are a few problems with this change :
 
 * Root tablet code assumes all files on one volume. I think these changes 
could break root tablet when there are multiple volumes.
 * A new tablet directory name is chosen for each new file and I don't think 
these will ever be garbage collected properly. Ideally tablets would use the 
same dir name across volumes.   For example a tablet with two files could look 
like :  `<vol1>/tables/5/t-000000e/F0000002.rf` and 
`<vol2>/tables/5/t-000000e/F0000003.rf`.  Notice that `t-000000e` is used 
across volumes.
 * Need to remove directory column from metadata table on upgrade
 * Need to remove code using directory column or repurpose column to contain 
tablet dir name.
 * Code that cleans up unused tmp files on tablet load may not inspect all 
volumes.

Addressing these issues would be involved.  If you would like some help making 
the changes, let me know.  I would like to see this over change made so that 
tablets can have files on different dirs.

[ Full content available at: https://github.com/apache/accumulo/pull/642 ]
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