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HDFS-1081 optimized the number of block access tokens (BATs) created in a single call to getBlockLocations, as this is an expensive operation. However, that JIRA put off another optimization which was then made possible, which is to just send a single block access token across the wire (and maintain a single BAT on the client side). This JIRA is for implementing that optimization. Since a single BAT is generated for all the blocks, we just write that single BAT to the wire, rather than writing n BATs for n blocks, as is currently done. This turns out to be a useful optimization for files with very large numbers of blocks, as the new lone BAT is much larger than was a BAT previously.