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segment:
	block 0: header: ifile inode location
			 seg_usage inode location.
			 next segment header location.
			 size (in blocks)
			 start of block->ino map

	block 1: (every 8th): SB
--------------
during data change:
	find new block location
		if in "current" segment, quit
	change seg usage bit for "current" segment (calls datachange)
	change seg usage bit for "old" segment (calls datachange)

-----------------
what about cleaning? must find owners of blocks we want to move

4 blocks: can describe all blocks in segment.
	first few bit fields are inode/indirect classification
	array of ino #s
valid info found by mapping usage bitmap to inode table in segment