Check status:

# cat /proc/mdstat
Personalities : [raid1]
md125 : active raid1 sdb3[1]
      450747264 blocks super 1.2 [2/1] [_U]

md126 : active raid1 sdb2[1]
      36949888 blocks super 1.2 [2/1] [_U]
      bitmap: 1/1 pages [4KB], 65536KB chunk

md127 : active raid1 sdb1[1]
      524224 blocks super 1.0 [2/1] [_U]

unused devices: <none>

/dev/sda seems missing, check dmesg:

# dmesg |grep sda
[2212380.750253] end_request: I/O error, dev sda, sector 2048
[2212380.750263] md/raid1:md127: Disk failure on sda1, disabling device.

Or with smartctl:

# smartctl /dev/sda
smartctl 6.2 2013-07-26 r3841 [x86_64-linux-3.10.0-229.1.2.el7.x86_64] (local build)
Copyright (C) 2002-13, Bruce Allen, Christian Franke, www.smartmontools.org

Smartctl open device: /dev/sda failed: No such device

Remove the drive from the array if it isn't already removed. Seems md already diabled my drive:

messages-20150504.gz:May  3 00:41:06 kvm2 kernel: md/raid1:md126: Disk failure on sda2, disabling device.
messages-20150504.gz:May  3 00:41:06 kvm2 kernel: md/raid1:md125: Disk failure on sda3, disabling device.
messages-20150504.gz:May  3 01:02:01 kvm2 kernel: md/raid1:md127: Disk failure on sda1, disabling device.

Check the serial number for the good drive:

]# smartctl -a /dev/sdb |grep -i serial
Serial Number:    9WJ0MGBM

Replace the failed drive, double check the serial number on the drive.

Copy boot sector / partition table to new drive:

sfdisk -d /dev/sdb | sfdisk /dev/sda

Put new drive in array:

mdadm /dev/md125 -a /dev/sda3
mdadm /dev/md126 -a /dev/sda2
mdadm /dev/md127 -a /dev/sda1
grub2-install --recheck /dev/sda

Watch /proc/mdstat:

# cat /proc/mdstat 
Personalities : [raid1] 
md125 : active raid1 sda3[2] sdb3[1]
      450747264 blocks super 1.2 [2/1] [_U]
      [>....................]  recovery =  0.0% (121856/450747264) finish=6938.0min speed=1082K/sec
      
md126 : active raid1 sda2[2] sdb2[1]
      36949888 blocks super 1.2 [2/1] [_U]
        resync=DELAYED
      bitmap: 1/1 pages [4KB], 65536KB chunk

md127 : active raid1 sda1[2] sdb1[1]
      524224 blocks super 1.0 [2/1] [_U]
        resync=DELAYED
      
unused devices: <none>

Show current min/max limit:

sysctl dev.raid.speed_limit_min
sysctl dev.raid.speed_limit_max

Increase speed:

sysctl -w dev.raid.speed_limit_min=100000