This file tells of raid status, which raid personalities are compiled in the kernel, which raid devices are configured and active and whether anything has gone wrong, such as a disk failure. You can also monitor resync progress and estimate resync finish time.

Example output:

Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md0 : active raid1 hdb2[1] hda3[0]
      107201664 blocks [2/2] [UU]
      [=====>...............]  resync = 28.1% (30158784/107201664) finish=37.4min speed=34274K/sec

unused devices: <none>