Wish to specify the size of the rpool datapool/partition during installation

[BertN45]

I like to be able to specify the size of the rpool partition during installation at the moment I select ZFS, you could ask to specify the size or to use the whole disk. The remainder of the disk could be left empty. I like that feature for the following reasons:

- Many users like me will have a SSD and a HDD, limiting the size of the rpool partition will allow them to use part of the SSD as a L2ARC/ZIL for the HDD.
- My current laptop HDD is 2TB and a rpool partition of 2TB makes Ubuntu very slow, when after a while the HDD gets fragmented. Due to OS updates the OS gets scattered all over the HDD resulting in considerable longer HDD seek times. A smaller rpool datapool (I would choose say 10GB) would speed up the OS on the HDD.
- It would allow us to use the gnome-disk-utility as simple backup of last resort for the EFI, rpool and bpool partitions. For my laptop and desktop I could store those two backups for example on an old 160GB USB HDD.
- I don't like to store my VMs in the USERDATA dataset nor in another own dataset in the rpool datapool. However on the desktop I like them on the nvme-SSD, but preferably in an own datapool with own properties.

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 5.13.0-19.19-generic 5.13.14
Uname: Linux 5.13.0-19-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu70
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Sun Oct 17 12:25:05 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
InstallationDate: Installed on 2021-10-09 (7 days ago)
InstallationMedia: Ubuntu 21.04 "Hirsute Hippo" - Release amd64 (20210420)
SourcePackage: ubiquity
Symptom: installation
UpgradeStatus: Upgraded to impish on 2021-10-14 (3 days ago)