I am running Ubuntu with ZFS on root, and my `bpool` seems to be almost full.

Output of `zpool list bpool`:

```
NAME    SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
bpool  1.88G  1.69G   191M        -         -    60%    90%  1.00x    ONLINE  -

```

Output of `df -h /boot`:

```
Filesystem                                        Size  Used Avail Use% Mounted on
bpool/BOOT/ubuntu_s2c09x                          367M  303M   64M  83% /boot

```

This is causing problems with completing the installation of kernel updates, e.g. running `sudo apt upgrade`:

```
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
2 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] 
Requesting to save current system state
ERROR couldn't save system state: Minimum free space to take a snapshot and preserve ZFS performance is 20%.
Free space on pool "bpool" is 10%.
Please remove some states manually to free up space. 
Setting up linux-firmware (1.197.5) ...
update-initramfs: Generating /boot/initrd.img-5.11.0-41-generic
I: The initramfs will attempt to resume from /dev/nvme0n1p2
I: (UUID=abdb40da-2d0b-4a45-b024-1c7245ea27ea)
I: Set the RESUME variable to override this.
Error 24 : Write error : cannot write compressed block 
E: mkinitramfs failure cpio 141 lz4 -9 -l 24
update-initramfs: failed for /boot/initrd.img-5.11.0-41-generic with 1.
dpkg: error processing package linux-firmware (--configure):
 installed linux-firmware package post-installation script subprocess returned error exit status 1
Setting up linux-image-5.11.0-34-generic (5.11.0-34.36) ...
Processing triggers for linux-image-5.11.0-34-generic (5.11.0-34.36) ...
/etc/kernel/postinst.d/dkms:
 * dkms: running auto installation service for kernel 5.11.0-34-generic
Error! Your kernel headers for kernel 5.11.0-34-generic cannot be found.
Please install the linux-headers-5.11.0-34-generic package,
or use the --kernelsourcedir option to tell DKMS where it's located
   ...done.
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-5.11.0-34-generic
I: The initramfs will attempt to resume from /dev/nvme0n1p2
I: (UUID=abdb40da-2d0b-4a45-b024-1c7245ea27ea)
I: Set the RESUME variable to override this.
Error 24 : Write error : cannot write compressed block 
E: mkinitramfs failure cpio 141 lz4 -9 -l 24
update-initramfs: failed for /boot/initrd.img-5.11.0-34-generic with 1.
run-parts: /etc/kernel/postinst.d/initramfs-tools exited with return code 1
dpkg: error processing package linux-image-5.11.0-34-generic (--configure):
 installed linux-image-5.11.0-34-generic package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 linux-firmware
 linux-image-5.11.0-34-generic
ZSys is adding automatic system snapshot to GRUB menu

```

I've tried destroying some bpool snapshots, but `zfs list -t snapshot | grep bpool` doesn't list any over 100K. I have also tried removing some kernels, see output of `dpkg -l linux-{image,headers}-"[0-9]*"`:

```
un  linux-headers-3.0               <none>         <none>       (no description available)
un  linux-headers-5.10.0-1045-oem   <none>         <none>       (no description available)
un  linux-headers-5.11.0-34-generic <none>         <none>       (no description available)
un  linux-headers-5.11.0-35-generic <none>         <none>       (no description available)
un  linux-headers-5.11.0-36-generic <none>         <none>       (no description available)
un  linux-headers-5.11.0-37-generic <none>         <none>       (no description available)
un  linux-headers-5.11.0-38-generic <none>         <none>       (no description available)
ii  linux-headers-5.11.0-40         5.11.0-40.44   all          Header files related to Linux kernel version 5.11.0
ii  linux-headers-5.11.0-40-generic 5.11.0-40.44   amd64        Linux kernel headers for version 5.11.0 on 64 bit x86 SMP
ii  linux-headers-5.11.0-41         5.11.0-41.45   all          Header files related to Linux kernel version 5.11.0
ii  linux-headers-5.11.0-41-generic 5.11.0-41.45   amd64        Linux kernel headers for version 5.11.0 on 64 bit x86 SMP
un  linux-headers-686-pae           <none>         <none>       (no description available)
ic  linux-image-5.10.0-1045-oem     5.10.0-1045.47 amd64        Signed kernel image oem
un  linux-image-5.11.0-27-generic   <none>         <none>       (no description available)
iF  linux-image-5.11.0-34-generic   5.11.0-34.36   amd64        Signed kernel image generic
rc  linux-image-5.11.0-35-generic   5.11.0-35.37   amd64        Signed kernel image generic
rc  linux-image-5.11.0-36-generic   5.11.0-36.40   amd64        Signed kernel image generic
rc  linux-image-5.11.0-37-generic   5.11.0-37.41   amd64        Signed kernel image generic
rc  linux-image-5.11.0-38-generic   5.11.0-38.42   amd64        Signed kernel image generic
ii  linux-image-5.11.0-40-generic   5.11.0-40.44   amd64        Signed kernel image generic
ii  linux-image-5.11.0-41-generic   5.11.0-41.45   amd64        Signed kernel image generic

```

The oldest currently installed one seems to be linux-image-5.11.0-34-generic, however `sudo apt purge linux-image-5.11.0-34-generic` suggests that it wants to also install `linux-image-unsigned-5.11.0-34-generic`, taking up more space.

What else can I try? Is it conceivably an option to resize/grow my bpool? I am pretty new to using ZFS.

Edit: I did some more investigating. `zfs list bpool/BOOT/ubuntu_s2c09x -o space -r` reports

```
NAME                                                             AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
bpool/BOOT/ubuntu_s2c09x                                          229M  1.51G     1.22G    303M             0B         0B
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_weekly-2021-11-19-1344        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-21-1339         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-22-1336         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-23-1340         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-24-1335         -    56K         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-25-1339         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-26-1338         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_weekly-2021-11-26-1343        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-27-1338         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-28-1336         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-29-1338         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-11-30-1340         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-01-1340         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-02-1340         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-03-1339         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_weekly-2021-12-03-1344        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-04-1337         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-05-1338         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-06-1338         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-07-1339         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-08-1337         -    80K         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-09-0645         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_a2u0ly                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-10-1336         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_weekly-2021-12-10-1341        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-11-1339         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-12-1335         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_omna1o                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_5q7106                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-13-1336         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_fwalez                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-14-1336         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_dsm4th                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-15-1335         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_4t769s                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-16-1336         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_z8aa09                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-17-1339         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_weekly-2021-12-17-1344        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_dqsnib                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_7w0l6s                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_323q08                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@autozsys_gqbsa5                             -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-18-1338         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-19-1339         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-0217        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-0317        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-0417        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-0517        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-0617        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-0717        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-0817        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-0917        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1017        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1117        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1217        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1317        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_daily-2021-12-21-1337         -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1417        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1517        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1617        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1717        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1817        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-1917        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-2017        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-2117        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-2217        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-21-2317        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-22-0017        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_frequent-2021-12-22-0100      -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_frequent-2021-12-22-0115      -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_hourly-2021-12-22-0117        -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_frequent-2021-12-22-0130      -     0B         -       -              -          -
bpool/BOOT/ubuntu_s2c09x@zfs-auto-snap_frequent-2021-12-22-0145      -     0B         -       -              -          -

```

I don't understand how it can claim that 1.22 GB are used by snapshots, but none of the listed bpool snapshots are using any space?