Do MBR partition numbers need to be contiguous?

Question

I am partitioning an external 1TB HDD for a small embedded Linux system. I want to encrypt the swap partition. According to the cryptsetep FAQ, you need to use kernel device names (/dev/sda, etc) in /etc/crypttab:

Specifying it directly by UUID does not work, unfortunately, as the UUID is part of the swap signature and that is not visible from the outside due to the encryption and in addition changes on each reboot.

This may become a problem if I attach/rearrange drives later and the device name changes. For example, say the swap is on /dev/sda3. Then I attach a different drive which becomes /dev/sda, pushing the original drive to /dev/sdb. If there exists a third partition on the new drive (now called sda3), it will try to encrypt that drive and use it as swap.

One option given is to make sure sure the partition number is not present on additional disks. So, finally, my question:

Can I use non-contiguous partition numbers? Will they persist across reboots? In other words, could I do this? Note the gap between sda4 and sda8:

/dev/sda1    primary    /boot
/dev/sda2    primary    /
/dev/sda3    primary    /home
/dev/sda4    extended
/dev/sda8    swap (encrypted)

If so, I would feel pretty safe about never seeing sda8 on any other drive.

Answer 1

Partition numbers cannot conflict. Physically cannot.

The partitions are recorded in a Partition Table, special place in the 0-block of the disk. These records are not a named records, they are placed in an array. The index in that array (plus one) later become a number in the list of partitions you see in terminal. Read wiki for example: https://en.wikipedia.org/wiki/Disk_partitioning

And yes, Partition Table can have empty cells. Ot is just an indexed array. Any record in it can have a zero for Partition Type and all tools would know that this record is empty.