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.
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md0 : active raid1 hdb2 hda3
107201664 blocks [2/2] [UU]
[=====>...............] resync = 28.1% (30158784/107201664) finish=37.4min speed=34274K/sec
unused devices: <none>