Implement pools trimming service

Trim the boot and root pools, via SystemD service, in exactly the same way the `fstrim` service does.

install-zfs.sh

@@ -1141,6 +1141,45 @@ function update_zed_cache_Debian {
   chroot_execute "sed -Ei 's|$c_installed_os_data_mount_dir/?|/|' /etc/zfs/zfs-list.cache/$v_rpool_name"
 }
 
+# We don't care about synchronizing with the `fstrim` service for two reasons:
+#
+# - we assume that there are no other (significantly) large filesystems;
+# - trimming is fast (takes minutes on a 1 TB disk).
+#
+# The code is a straight copy of the `fstrim` service.
+#
+function configure_pools_trimming {
+  print_step_info_header
+
+  chroot_execute "cat > /lib/systemd/system/zfs-trim.service << UNIT
+[Unit]
+Description=Discard unused ZFS blocks
+ConditionVirtualization=!container
+
+[Service]
+Type=oneshot
+ExecStart=/sbin/zpool trim $v_bpool_name
+ExecStart=/sbin/zpool trim $v_rpool_name
+UNIT"
+
+  chroot_execute "  cat > /lib/systemd/system/zfs-trim.timer << TIMER
+[Unit]
+Description=Discard unused ZFS blocks once a week
+ConditionVirtualization=!container
+
+[Timer]
+OnCalendar=weekly
+AccuracySec=1h
+Persistent=true
+
+[Install]
+WantedBy=timers.target
+TIMER"
+
+  chroot_execute "systemctl daemon-reload"
+  chroot_execute "systemctl enable zfs-trim.timer"
+}
+
 function configure_remaining_settings {
   print_step_info_header
 
@@ -1236,6 +1275,7 @@ distro_dependent_invoke "install_and_configure_bootloader"
 sync_efi_partitions
 configure_boot_pool_import
 distro_dependent_invoke "update_zed_cache" --noforce
+configure_pools_trimming
 configure_remaining_settings
 
 prepare_for_system_exit