Iotop tool reports that
md2_raid1 takes HDD IO up to 99 %.
The server's HDD becomes busy and it's impossible to do any operation on it.
Here is the output of
Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] md2 : active raid1 sda3 sdb3 1462766336 blocks [2/2] [UU] md1 : active raid1 sda2 sdb2 264960 blocks [2/2] [UU] md0 : active raid1 sda1 sdb1 2102464 blocks [2/2] [UU] unused devices: <none>