swap partition may come up read-only:

What device is /dev/sda?

root@vamos2:~# smartctl -i /dev/sda
smartctl 6.4 2014-10-07 r4002 [x86_64-linux-3.16.0-4-amd64] (local build)
Copyright (C) 2002-14, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     Western Digital Red (AF)
Device Model:     WDC WD20EFRX-683UZNO
Serial Number:    WD-DEADBEEF
LU WWN Device Id: 5 0014ee 2b73f71d9
Firmware Version: 82.00A82
User Capacity:    2,000,398,934,016 bytes [2.00 TB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
Rotation Rate:    5400 rpm
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   ACS-2 (minor revision not indicated)
SATA Version is:  SATA 3.0, 6.0 Gb/s (current: 6.0 Gb/s)
Local Time is:    Mon Jan 23 14:06:15 2017 CET
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

md1 : active (auto-read-only) raid1 sda3[3] sdb3[4](W) sdc3[2]
      7811464 blocks super 1.2 [3/3] [UUU]
      bitmap: 0/1 pages [0KB], 65536KB chunk

This is not a problem, and can be fixed with mdadm /dev/md1 –readwrite

Array rebuilding:

root@vamos2:~# cat /proc/mdstat 
Personalities : [raid1] 
md2 : active raid1 sdb4[3] sda4[4] sdc4[2]
      1926164748 blocks super 1.2 [3/2] [UU_]
      [=>...................]  recovery =  6.4% (123824128/1926164748) finish=313.1min speed=95918K/sec
      bitmap: 1/15 pages [4KB], 65536KB chunk

md1 : active raid1 sda3[3] sdb3[4](W) sdc3[2]
      7811464 blocks super 1.2 [3/3] [UUU]
      bitmap: 0/1 pages [0KB], 65536KB chunk

md0 : active raid1 sda2[3] sdb2[4](W) sdc2[2]
      19530154 blocks super 1.2 [3/3] [UUU]
      bitmap: 1/1 pages [4KB], 65536KB chunk

unused devices: <none>

The data array didn't come up. I started it with mdadm –assemble /dev/md2 /dev/sda4 /dev/sdc4