What is the purpose of an bpool partition in ZFS and what does it contain?

Question

With an installation of Ubuntu-Cinnamon with ZFS as file system, the following partitions are created:

/dev/sda1  (efi)
/dev/sda2  (swap)
/dev/sda3  (bpool)
/dev/sda4  (rpool for data)

What is the purpose of an bpool partition with ZFS and what does it contain?

Answer 1

Per this article from Tech Republic about How to Manage ZFS Pools in Ubuntu 19.10:

One thing you should understand how to do is the management of your ZFS Pools. ZFS is capable of managing data that spans across devices. ZFS uses virtual storage pools, called zpools. A zpool can contain numerous directories. For example, on a Ubuntu Desktop 19.10 installation (with ZFS support added), there are two basic pools:

• bpool is the boot pool and stores all boot-related directores
• rpool is the root pool and stores everything under / (minus /boot)

bpool is equivalent to a non-ZFS /boot partition containing the kernel images, GRUB bootloader configurations, etc. kept independent from the root disk partition. (LVM installations create a separate /boot partition so that it boots properly from an on-disk partition). Anything you see in /boot when you examine your system filestructure is what's in bpool.

Concurrently, rpool is where all your data on system resides. Your installation, your user data, etc. all lives in rpool.

(This standard continues despite the article referring to a now-end-of-life release of Ubuntu, because it's still accurate definitions of both bpool and rpool)

You can read up about this also in the technical details within the OpenZFS documentation on Ubuntu 22.04 Root on ZFS for more technical implementation details.

This should also be reflected in the output of df on the command line, and should show you which pools are mapped to which directory locations in your filesystem structure on disk. Namely, that /boot is in bpool, and / (root partition) is in rpool.