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Proxmox: Switching from legacy boot when there is no space for ESP partition

Date: Author:

/u/kyle0r⁠

Published: 2024-01-07 Last updated: 2024-01-09

Also posted on proxmox forums:

[TUTORIAL] - Switching from legacy boot when there is no space for...⁠

🏆 A poster on the thread shared a shorter set of steps utilising ZFS vdev child device evacuation feature.

Disclaimer

I’ve tried to include some guardrails and warnings in this content. The nature of some of the steps is destructive. This content comes with no warranties or guarantees. It is your responsibility to:

Create a tested and verified backup and a rollback strategy in case something goes wrong.

Check that the commands you execute do what you expect, prepending echo to a command will give you a preview of what your shell will interpret.

Consider performing these steps on a lab/vm setup before performing them on a critical system.

Background

Why create this content? I’m capturing this for my future reference, and perhaps others can benefit from the content... to help solve their own similar challenges.

💡 The spare $boot_temp drive I used in this guide happened to be a spare CT500MX that I had in cold storage, which is identical to my rpool vdev children. However, I could have used any bootable drive, including a USB stick. For example, the steps captured here could be adapted to work with a USB flash memory drive.

💡 The steps captured here could be adapted to perform a general proxmox rpool migration from one system or drive to another.

This section⁠

specifically.

Prior to attempting this, I had spent a not insignificant amount of time thinking about how to solve this challenge and the rough steps required. See the

flowchart⁠

. No doubt this little journey has given me a few extra grey beard hairs 😉.

As of writing the proxmox VE is now on release 8. Since a few major releases of Proxmox VE, the release notes have strongly suggested switching the bootloader from legacy-boot to the proxmox-boot-tool. Sounds good in principle and the steps are well documented

here⁠

. However the steps do not cover the case where there is no space to create an ESP partition (as per the proxmox v4 installer partition scheme), so this documentation captures what I did to switch in the case where an ESP partition cannot be created on the existing rpool drives.

I had procrastinated doing this work for a while (at least a few major proxmox releases), and finally bit the bullet after recently gaining more experience and confidence with ZFS internals, and recovering and debugging ZFS systems (should something go wrong).

In the process of completing this work I have become much more familiar with zdb and the process of importing, rewinding, and exporting zpools. I’ve captured some of the relevant knowledge in my

OpenZFS cheatsheet⁠

Inspiration:

@whytf⁠

shared steps

here⁠

for using proxmox-boot-tool with a USB stick to boot an existing proxmox ZFS installation.

Problem statement, challenges and objectives

“So this was a legacy system with grub without an ESP? That is known to be problematic (it's basically a timebomb that can explode at any write to /boot), which is why we switched to a different setup.” ~Fabian - Proxmox Staff Member [

citation link⁠

Background: Related to Fabian's quote, which comes from an indirectly related forum post... The system originated from a Proxmox v4 installer that used the legacy boot approach, where grub loaded the kernel and co from /boot on the ZFS rpool. This setup suffered from fragile compatibility between grub and ZFS. zpool upgrades were known to create unbootable systems with this setup.

Read more⁠

Challenges switching to proxmox-boot-tool: The proxmox v4 installer used a partition for the ZFS rpool instead of whole-disk mode. ZFS relies on the partition offset to locate ZFS objects and their blocks on the vdev children. So moving or shrinking the partition sector offsets of the rpool mirror to make space for a vfat/efi partition was not possible. i.e. creating a suitable vfat/efi partition was not possible without breaking ZFS.

This means the following approach would not work:

https://pve.proxmox.com/wiki/ZFS:_Switch_Legacy-Boot_to_Proxmox_Boot_Tool⁠

⁠

Problems:

zpool upgrade on the rpool - high risk the system becomes unbootable with the legacy approach.

Boot fragility when modifying /boot on the rpool e.g. kernel updates.

proxmox-boot-tool could not be used without an ESP partition.

Objectives:

Keep the rpool and existing proxmox install - avoid fresh install

Resolve fragile boot issues - migrate /boot from ZFS to vfat partition

Start using proxmox-boot-tool

Be able to zpool upgrade the rpool without fear of breaking the system (boot)

Lay the groundwork for later switching from legacy BIOS mode to UEFI mode and from grub to systemd.

Infographic: visualisation of AS-IS and TARGET setup

⁠

⁠

Flow chart of the steps

I created the first revision of this flowchart to get my head around the steps prior to committing to the effort of actually doing the steps. What you see shared here is the latest revision, which reflects the actual flow and benefits from some tweaks made during the writing process.

💡 You might wish to study this flowchart rather than reading the actual verbose steps to get an idea if any of this content can be useful to your scenario.

⁠

⁠

Drive overview

💡 The name column in the following table is the name of the shell variable used in this guide.

name

path

note

boot_temp

/dev/disk/by-id/ata-CT500MX500SSD1_2045E4____4A

spare drive. will be used as a temporary boot drive.

rpool_new

/dev/disk/by-id/ata-CT500MX500SSD1_1934E2____65

rpool mirror vdev child. this drive will be taken out of the mirror to create a new rpool.

rpool_old

/dev/disk/by-id/ata-CT500MX500SSD1_2045E4____6C

rpool mirror vdev child, this drive will be the rollback if something goes wrong. Eventually it will be attached to the new rpool.

There are no rows in this table

⁠

The steps

Prerequisites

As a prerequisite, I strongly recommend that you study the following two proxmox documents carefully and become familiar with them:

⁠

Host Bootloader - Proxmox VE⁠

⁠

ZFS: Switch Legacy-Boot to Proxmox Boot Tool - Proxmox VE⁠

⁠

BIOS legacy OR UEFI? grub OR systemd? Secure boot?

💡 The system in my procedures was using BIOS in legacy mode, non-secure boot and grub for the bootloader.

Its important that you are aware and cognitive of the following points... and that you adjust your procedure accordingly:

Is your BIOS set to legacy OR UEFI?

Is your BIOS set to secure boot? [

docs link⁠

]

Is your bootloader grub OR systemd?

Out of scope: Switching from BIOS legacy to UEFI mode?

This is outside of the scope of this guide/procedure, but here is some things to consider:

In related info/research: there is an interesting thread on the proxmox forums:

Benefits of using UEFI BIOS vs. Legacy BIOS⁠

and another thread

Legacy BIOS to UEFI mode⁠

BIOS, firmware, IPMI can be updated through UEFI with https://fwupd.org/

It is important to consider if/when switching from legacy boot to UEFI, that all things in the boot chain need to be EFI ready, that includes BIOS, HBA’s / storage controllers, and the operating system. The same applies if switching a virtual machine / kvm from legacy BIOS to UEFI. The vm operating system will very likely need some UEFI preparation to make the switch.

Setting up the bootloader on the temporary boot drive

In this stage, the objective is to get the system booting via the proxmox-boot-tool and retire the legacy boot approach.

# set the $boot_temp variable, the first drive we will work with

boot_temp=/dev/disk/by-id/ata-CT500MX500SSD1_2045E4____4A

# make a note and be congnitive of the logical and physical sector sizes of the drive

# ⚠ this may be different for your drive ⚠

gdisk -l $boot_temp |grep ^Sector

output:

Sector size (logical/physical): 512/4096 bytes

# check the blockdev report too

# ⚠ this may be different for your drive ⚠

blockdev --report $boot_temp

output:

RO RA SSZ BSZ StartSec Size Device

rw 256 512 4096 0 500107862016 /dev/disk/by-id/ata-CT500MX500SSD1_2045E4____4A

# The --report shows SSZ = Sector Size, and BSZ = Block Size.

# These values will match the output from gdisk.

# The logical SSZ Sector Size: The sector size used by the operating system to address the device.

# The physical BSZ Block Size: The sector size used by the drive firmware to address the device.

# The CT500MX drives in this guide are 512e/4kn e for emulated, n for native.

# 💡 gdisk partitions drives in logical sectors.

# The physical sector size is usually greater than or equal to the logical sector size.

# For illustration, the CT500MX drives used in this guide have 976,773,168 x 512 logical sectors = 500,107,862,016 bytes, when rounded = 466GiB = 500GB

# The drives have 122,096,646 4KiB physical sectors.

# FYI: some modern, typically enterprise drives use 4kn native sectors - 4k logical and physical sectors. You can read more about 4kn

here⁠⁠⁠

here⁠

and

here⁠⁠⁠⁠⁠⁠⁠⁠

# make a note and be congnitive of the drives aliginment

# ⚠ this may be different for your drive/system ⚠

gdisk -l $boot_temp |grep aligned

output:

Partitions will be aligned on 2048-sector boundaries

# reference: 2048 * 512 = 1048576 bytes = 1 MiB sector boundaries

# set up the partitions on the new boot drive, paying attention to the block/sector alignment.

# ⚠ WARNING: misaligned partitions can adversely affect drive and system performance. In the case of SSDs, their lifespan may be reduced.

# Optimal partition alignment will mitigate performance issues and penalties such as Read/Modify/Write (RMW).

# knowledge on alignment

here⁠⁠⁠

and

here⁠

# 💡 a partition START and SIZE in BYTES should be devisable by the logical and physical sector size in BYTES

# 💡 partitions should only start on (be algined with) a drives sector boundaries, for the CT500MX drives this means 2048 sectors = 1 MiB

gdisk $boot_temp

# I created a +1M EF02 BIOS boot partition, starting at sector 2048.

# I created a +2G EF00 EFI system partition, following partition 1: start sector 4096.

# the $boot_temp drive doesn't technically need a 3rd partition, but since it's identical to the other CT500MX drives, I can create the 3rd partition now and simplify the steps required later.

# For ZFS rpool - I created a code:8300 Linux filesystem and left 1GiB of free space at the end of the drive.

# In my case the partition size was 970479616 sectors / 496885563392 bytes.

# here is $boot_temp drive and partition info in SECTORS

parted $boot_temp unit s print

output:

Model: ATA CT500MX500SSD1 (scsi)

Disk /dev/sdf: 976773168s # drive size in SECTORS

Sector size (logical/physical): 512B/4096B

Partition Table: gpt

Disk Flags:

Number Start End Size File system Name Flags

1 2048s 4095s 2048s zfs BIOS boot partition bios_grub

2 4096s 4194304s 4190209s fat32 EFI system partition boot, esp

3 4196352s 974675967s 970479616s zfs Linux filesystem

# 💡 Notice that this drive is aligned on 2048-sector boundaries.

# Note that partition 1 starts at sector 2048, and note that the delta between the END of partition 2 and the START of partition 3 is 2048 sectors. This is the correct 1MiB sector boundaries.

# here is $boot_temp drive and partition info in BYTES

parted $boot_temp unit B print

output:

Model: ATA CT500MX500SSD1 (scsi)

Disk /dev/sdf: 500107862016B # drive size in BYTES

Sector size (logical/physical): 512B/4096B

Partition Table: gpt

Disk Flags:

Number Start End Size File system Name Flags

1 1048576B 2097151B 1048576B zfs BIOS boot partition bios_grub

2 2097152B 2147484159B 2145387008B fat32 EFI system partition boot, esp

3 2148532224B 499034095615B 496885563392B zfs Linux filesystem