After moving from a hard drive to an SSD on my work laptop, I decided to keep the hard drive spinning and use it as a backup for the SSD.

With the following setup, I can pull the SSD out of my laptop and it should still boot up normally with all of my data on the hard drive.

Manually setting up an encrypted root partition

Before setting up the synchronization between the two drives, I had to replicate the partition setup.

I used fdisk, cfdisk and gparted to create the following partitions:

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *        2048      499711      248832   83  Linux
/dev/sdb2          501760   500117503   249807872    5  Extended
/dev/sdb5          503808   500117503   249806848   83  Linux

and then loosely followed these instructions to create an encrypted root partition on /dev/sdb5:

$ cryptsetup luksFormat /dev/sdb5
$ cryptsetup luksOpen /dev/sdb5 sdb5_crypt
$ pvcreate /dev/mapper/sdb5_crypt
$ vgcreate akranes2 /dev/mapper/sdb5_crypt
$ vgchange -a y akranes2
$ lvcreate -L247329718272B -nroot akranes2
$ lvcreate -L8468299776B -nswap_1 akranes2
$ mkfs.ext4 -m1 /dev/akranes2/root

Finally, I added the new encrypted partition to the list of drives to bring up at boot time by looking up its UUID:

$ cryptsetup luksUUID /dev/sdb5

and creating a new entry for it in /etc/crypttab.

Copying the boot partition

Setting up the boot partition was much easier because it's not encrypted. All that was needed was to format it and then copy the files over:

$ mkfs.ext2 /dev/sdb1
$ mount /dev/sdb1 /mnt/boot
$ cp -a /boot/* /mnt/boot/

The only other thing to remember is to install grub on the boot loader of that drive. On modern Debian systems, that's usually just a matter of running dpkg-reconfigure grub-pc and adding the second drive (/dev/sdb in my case) to the list of drives to install grub on.

Sync scripts

To keep the contents of the SSD and the hard drive in sync, I set up a regular rsync of the root and boot partitions using the following mount points (as defined in /etc/fstab):

/dev/mapper/akranes-root /           ext4    noatime,discard,errors=remount-ro 0       1
/dev/mapper/akranes2-root /mnt/root  ext4    noatime,errors=remount-ro,nofail  0       2
UUID=0b9109d0-... /boot              ext2    defaults                          0       2
UUID=6e6f05fb-... /mnt/boot          ext2    defaults,nofail                   0       2

I use this script (/usr/local/sbin/ssd_boot_backup) for syncing the boot partition:

if [ ! -e /mnt/boot/hdd.mounted ] ; then
    echo "The rotating hard drive is not mounted in /mnt/boot."
    exit 1
if [ ! -e /boot/ssd.mounted ] ; then
    echo "The ssd is not the boot partition"
    exit 1
nocache nice ionice -c3 rsync -aHx --inplace --delete --exclude=/boot/ssd.mounted --exclude=/boot/hdd.mounted --exclude=/boot/lost+found/* /boot /mnt/

and a similar one (/usr/local/sbin/ssd_root_backup) for the root partition:

if [ ! -e /mnt/root/hdd.mounted ] ; then
    echo "The rotating hard drive is not mounted in /mnt/root."
    exit 1
if [ ! -e /ssd.mounted ] ; then
    echo "The ssd is not the root partition"
    exit 1
nocache nice ionice -c3 rsync -aHx --delete --exclude=/dev/* --exclude=/proc/* --exclude=/sys/* --exclude=/tmp/* --exclude=/boot/* --exclude=/mnt/* --exclude=/lost+found/* --exclude=/media/* --exclude=/run/* --exclude=/scratch/* --exclude=/var/tmp/* --exclude=/ssd.mounted --exclude=/var/lock/* /* /mnt/root/

To ensure that each drive is properly mounted before the scripts run, I created empty ssd.mounted files in the root directory of each of the partitions on the SSD, and empty hdd.mounted files in the root directory of the hard drive partitions.

Note that the boot partition rsync command uses the --inplace option. That's to prevent No space left on device errors on due to the temporary file copying (the rsync default mode of operation) on small partitions.

Cron jobs

The sync scripts are run every couple of hours through this crontab:

10 */4 * * *                root    /usr/local/sbin/ssd_boot_backup
20 0,4,8,12,16,20 * * *     root    /usr/local/sbin/ssd_root_backup
20 2,6,10,14,18,22 * * *    root    /usr/bin/on_ac_power && /usr/local/sbin/ssd_root_backup

which includes a reduced frequency while running on battery to avoid spinning the hard drive up too much.