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mount options?  tflush: seconds sflush: segments
		timetillflush, segmentstillflush, idleflush?? -> best


what does a checkpoint do?
	write to superblocks:
		ifile inode block #
	cycle through superblocks.
	umount flushes current sb to all sbs.

segment roll-forward stops when:
	"next" segment has:
		- failed CRC
		- older data than previous segment

how do I get current time?
how do I calculate CRC?