If you look at the top of the file you'll see:

/*
 * Mirror one arena partition onto another.  
 * Be careful to copy only new data.
 */

Now if you combine this with some of the information from venti(8):

          The data log is logically split into sections called arenas,
          typically sized for easy offline backup (e.g., 500MB).  A
          data log may comprise many disks, each storing one or more
          arenas.  Such disks are called arena partitions. Arena par-
          titions are filled in the order given in the configuration.

My best guess is that this is an incremental backup tool, where you give it a 
source and destination arena partition and the code looks at both, figures out 
the deltas, then copies over only what is needed. The venti code is a bit 
inscrutable and I'm no venti guy so take this with a grain of salt.
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