I’ve been meaning to write about this for a while and since it’s come up recently (Write Intent Bitmaps, Software RAID) I figured now would be a good time. Basically what the write-intent bitmap does is keep track of which blocks are being used so that in the event that a resync is needed (power failure, kernel crash, disk temporarily offline) only the blocks that are out of sync are copied. This drastically improves resync times. Of course there is a downside.