Thanks for your reply (I expected this to go into the ether, since it seems to be a very common issue, with I/O, on this hardware platform). I did try switching from LZJB, to LZ4, and got slightly-better performance, and reliability, from at least testing with backing up a Windows 10 machine, using WSL, and Borg, over SSH - however, I can still lock the system up, after several runs, or get it to a stage where I can send commands, over an open SSH session, but spawning a new one fails, since it can't access my home directory (which is stored on the ZFS array). My last log, with Borg, is: [ 22.536219] bcmgenet: Skipping UMAC reset [ 22.537803] bcmgenet fd580000.ethernet: configuring instance for external RGMII [ 22.538029] bcmgenet fd580000.ethernet eth0: Link is Down [ 25.140217] new mount options do not match the existing superblock, will be ignored [ 26.751285] brcmfmac: brcmf_cfg80211_set_power_mgmt: power save enabled [ 27.651756] bcmgenet fd580000.ethernet eth0: Link is Up - 1Gbps/Full - flow control rx/tx [ 27.651790] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready [ 29.696209] Started bpfilter [ 29.701240] bpfilter: Loaded bpfilter_umh pid 3166 [ 29.858264] bridge: filtering via arp/ip/ip6tables is no longer available by default. Update your scripts to load br_netfilter if you need this. [ 30.379027] kauditd_printk_skb: 27 callbacks suppressed [ 30.379032] audit: type=1400 audit(1602433969.316:39): apparmor="STATUS" operation="profile_replace" info="same as current profile, skipping" profile="unconfined" name="/usr/bin/lxc-start" pid=3259 comm="apparmor_parser" [ 30.457589] audit: type=1400 audit(1602433969.396:40): apparmor="STATUS" operation="profile_replace" info="same as current profile, skipping" profile="unconfined" name="lxc-container-default" pid=3266 comm="apparmor_parser" [ 30.457603] audit: type=1400 audit(1602433969.396:41): apparmor="STATUS" operation="profile_replace" info="same as current profile, skipping" profile="unconfined" name="lxc-container-default-cgns" pid=3266 comm="apparmor_parser" [ 30.457620] audit: type=1400 audit(1602433969.396:42): apparmor="STATUS" operation="profile_replace" info="same as current profile, skipping" profile="unconfined" name="lxc-container-default-with-mounting" pid=3266 comm="apparmor_parser" [ 30.457629] audit: type=1400 audit(1602433969.396:43): apparmor="STATUS" operation="profile_replace" info="same as current profile, skipping" profile="unconfined" name="lxc-container-default-with-nesting" pid=3266 comm="apparmor_parser" [ 36.556387] Bluetooth: BNEP (Ethernet Emulation) ver 1.3 [ 36.556393] Bluetooth: BNEP filters: protocol multicast [ 36.556407] Bluetooth: BNEP socket layer initialized [ 74.720044] Adding 16777212k swap on /dev/zd0. Priority:-2 extents:1 across:16777212k SSFS [10030.116676] INFO: task z_rd_int:2154 blocked for more than 120 seconds. [10030.123432] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10030.130389] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10030.138388] z_rd_int D 0 2154 2 0x00000028 [10030.138403] Call trace: [10030.138424] __switch_to+0x104/0x170 [10030.138435] __schedule+0x30c/0x7c0 [10030.138444] schedule+0x3c/0xb8 [10030.138453] io_schedule+0x20/0x58 [10030.138466] rq_qos_wait+0x100/0x178 [10030.138475] wbt_wait+0xb4/0xf0 [10030.138485] __rq_qos_throttle+0x38/0x50 [10030.138497] blk_mq_make_request+0x128/0x610 [10030.138509] generic_make_request+0xb4/0x2d8 [10030.138519] submit_bio+0x48/0x218 [10030.138756] vdev_disk_io_start+0x670/0x9f8 [zfs] [10030.138977] zio_vdev_io_start+0xdc/0x2b8 [zfs] [10030.139197] zio_execute+0xac/0x110 [zfs] [10030.139414] vdev_queue_io_done+0x18c/0x2a0 [zfs] [10030.139632] zio_vdev_io_done+0xec/0x220 [zfs] [10030.139849] zio_execute+0xac/0x110 [zfs] [10030.139891] taskq_thread+0x2f8/0x570 [spl] [10030.139904] kthread+0xfc/0x128 [10030.139915] ret_from_fork+0x10/0x1c [10030.139928] INFO: task z_rd_int:2158 blocked for more than 120 seconds. [10030.146692] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10030.153619] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10030.161608] z_rd_int D 0 2158 2 0x00000028 [10030.161622] Call trace: [10030.161643] __switch_to+0x104/0x170 [10030.161655] __schedule+0x30c/0x7c0 [10030.161663] schedule+0x3c/0xb8 [10030.161673] io_schedule+0x20/0x58 [10030.161686] rq_qos_wait+0x100/0x178 [10030.161694] wbt_wait+0xb4/0xf0 [10030.161705] __rq_qos_throttle+0x38/0x50 [10030.161717] blk_mq_make_request+0x128/0x610 [10030.161729] generic_make_request+0xb4/0x2d8 [10030.161739] submit_bio+0x48/0x218 [10030.161965] vdev_disk_io_start+0x670/0x9f8 [zfs] [10030.162186] zio_vdev_io_start+0xdc/0x2b8 [zfs] [10030.162416] zio_nowait+0xd4/0x170 [zfs] [10030.162633] vdev_queue_io_done+0x1ec/0x2a0 [zfs] [10030.162859] zio_vdev_io_done+0xec/0x220 [zfs] [10030.163078] zio_execute+0xac/0x110 [zfs] [10030.163123] taskq_thread+0x2f8/0x570 [spl] [10030.163136] kthread+0xfc/0x128 [10030.163148] ret_from_fork+0x10/0x1c [10030.163160] INFO: task z_wr_iss:2163 blocked for more than 120 seconds. [10030.169921] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10030.176850] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10030.184833] z_wr_iss D 0 2163 2 0x00000028 [10030.184847] Call trace: [10030.184861] __switch_to+0x104/0x170 [10030.184871] __schedule+0x30c/0x7c0 [10030.184879] schedule+0x3c/0xb8 [10030.184887] io_schedule+0x20/0x58 [10030.184900] rq_qos_wait+0x100/0x178 [10030.184908] wbt_wait+0xb4/0xf0 [10030.184918] __rq_qos_throttle+0x38/0x50 [10030.184930] blk_mq_make_request+0x128/0x610 [10030.184942] generic_make_request+0xb4/0x2d8 [10030.184952] submit_bio+0x48/0x218 [10030.185175] vdev_disk_io_start+0x670/0x9f8 [zfs] [10030.185392] zio_vdev_io_start+0xdc/0x2b8 [zfs] [10030.185609] zio_nowait+0xd4/0x170 [zfs] [10030.185825] vdev_queue_io+0x23c/0x258 [zfs] [10030.186043] zio_vdev_io_start+0x94/0x2b8 [zfs] [10030.186260] zio_nowait+0xd4/0x170 [zfs] [10030.186476] vdev_mirror_io_start+0xa8/0x1b0 [zfs] [10030.186693] zio_vdev_io_start+0x248/0x2b8 [zfs] [10030.186910] zio_execute+0xac/0x110 [zfs] [10030.186948] taskq_thread+0x2f8/0x570 [spl] [10030.186960] kthread+0xfc/0x128 [10030.186971] ret_from_fork+0x10/0x1c [10030.186984] INFO: task z_wr_int:2172 blocked for more than 120 seconds. [10030.193743] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10030.200666] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10030.208657] z_wr_int D 0 2172 2 0x00000028 [10030.208671] Call trace: [10030.208685] __switch_to+0x104/0x170 [10030.208696] __schedule+0x30c/0x7c0 [10030.208710] schedule+0x3c/0xb8 [10030.208718] io_schedule+0x20/0x58 [10030.208730] rq_qos_wait+0x100/0x178 [10030.208742] wbt_wait+0xb4/0xf0 [10030.208753] __rq_qos_throttle+0x38/0x50 [10030.208764] blk_mq_make_request+0x128/0x610 [10030.208776] generic_make_request+0xb4/0x2d8 [10030.208787] submit_bio+0x48/0x218 [10030.209052] vdev_disk_io_start+0x670/0x9f8 [zfs] [10030.209273] zio_vdev_io_start+0xdc/0x2b8 [zfs] [10030.209503] zio_nowait+0xd4/0x170 [zfs] [10030.209720] vdev_queue_io_done+0x1ec/0x2a0 [zfs] [10030.209948] zio_vdev_io_done+0xec/0x220 [zfs] [10030.210166] zio_execute+0xac/0x110 [zfs] [10030.210212] taskq_thread+0x2f8/0x570 [spl] [10030.210225] kthread+0xfc/0x128 [10030.210236] ret_from_fork+0x10/0x1c [10030.210262] INFO: task txg_sync:2333 blocked for more than 120 seconds. [10030.217024] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10030.223955] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10030.231946] txg_sync D 0 2333 2 0x00000028 [10030.231960] Call trace: [10030.231975] __switch_to+0x104/0x170 [10030.231985] __schedule+0x30c/0x7c0 [10030.231993] schedule+0x3c/0xb8 [10030.232003] schedule_timeout+0x9c/0x190 [10030.232011] io_schedule_timeout+0x28/0x48 [10030.232050] __cv_timedwait_common+0x1a8/0x1f8 [spl] [10030.232084] __cv_timedwait_io+0x3c/0x50 [spl] [10030.232305] zio_wait+0x130/0x2a0 [zfs] [10030.232524] dsl_pool_sync+0x3fc/0x498 [zfs] [10030.232741] spa_sync+0x538/0xe68 [zfs] [10030.232997] txg_sync_thread+0x2c0/0x468 [zfs] [10030.233042] thread_generic_wrapper+0x74/0xa0 [spl] [10030.233057] kthread+0xfc/0x128 [10030.233068] ret_from_fork+0x10/0x1c [10030.233229] INFO: task borg:6275 blocked for more than 120 seconds. [10030.239643] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10030.246553] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10030.254520] borg D 0 6275 6274 0x00000000 [10030.254529] Call trace: [10030.254538] __switch_to+0x104/0x170 [10030.254545] __schedule+0x30c/0x7c0 [10030.254550] schedule+0x3c/0xb8 [10030.254556] schedule_timeout+0x9c/0x190 [10030.254561] io_schedule_timeout+0x28/0x48 [10030.254586] __cv_timedwait_common+0x1a8/0x1f8 [spl] [10030.254606] __cv_timedwait_io+0x3c/0x50 [spl] [10030.254741] zio_wait+0x130/0x2a0 [zfs] [10030.254871] dbuf_read+0x2e4/0xbb8 [zfs] [10030.255001] dmu_tx_check_ioerr+0x80/0xf0 [zfs] [10030.255130] dmu_tx_hold_zap_impl+0x84/0xc8 [zfs] [10030.255259] dmu_tx_hold_zap+0x50/0x60 [zfs] [10030.255387] dmu_tx_hold_sa_create+0x108/0x110 [zfs] [10030.255518] zfs_create+0x1d0/0x7d0 [zfs] [10030.255649] zpl_create+0xb8/0x178 [zfs] [10030.255657] lookup_open+0x4ec/0x6a8 [10030.255663] do_last+0x260/0x8c0 [10030.255669] path_openat+0x84/0x258 [10030.255675] do_filp_open+0x84/0x108 [10030.255682] do_sys_open+0x180/0x2b0 [10030.255689] __arm64_sys_openat+0x2c/0x38 [10030.255695] el0_svc_common.constprop.0+0x80/0x218 [10030.255700] el0_svc_handler+0x34/0xa0 [10030.255706] el0_svc+0x10/0x2cc [10030.255744] INFO: task du:441566 blocked for more than 120 seconds. [10030.262133] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10030.269041] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10030.277009] du D 0 441566 3688 0x00000000 [10030.277018] Call trace: [10030.277030] __switch_to+0x104/0x170 [10030.277036] __schedule+0x30c/0x7c0 [10030.277041] schedule+0x3c/0xb8 [10030.277048] schedule_timeout+0x9c/0x190 [10030.277053] io_schedule_timeout+0x28/0x48 [10030.277078] __cv_timedwait_common+0x1a8/0x1f8 [spl] [10030.277099] __cv_timedwait_io+0x3c/0x50 [spl] [10030.277236] zio_wait+0x130/0x2a0 [zfs] [10030.277367] dbuf_read+0x2e4/0xbb8 [zfs] [10030.277507] dmu_buf_hold+0xa4/0xd8 [zfs] [10030.277638] zap_lockdir+0x70/0xe8 [zfs] [10030.277774] zap_cursor_retrieve+0x164/0x2c8 [zfs] [10030.277905] zfs_readdir+0x124/0x398 [zfs] [10030.278040] zpl_iterate+0x50/0x80 [zfs] [10030.278049] iterate_dir+0x14c/0x1a0 [10030.278055] ksys_getdents64+0xac/0x1f8 [10030.278066] __arm64_sys_getdents64+0x28/0xd8 [10030.278072] el0_svc_common.constprop.0+0x80/0x218 [10030.278077] el0_svc_handler+0x34/0xa0 [10030.278083] el0_svc+0x10/0x2cc [10150.949406] INFO: task z_rd_int:2154 blocked for more than 241 seconds. [10150.956133] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10150.963040] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10150.970998] z_rd_int D 0 2154 2 0x00000028 [10150.971005] Call trace: [10150.971017] __switch_to+0x104/0x170 [10150.971024] __schedule+0x30c/0x7c0 [10150.971027] schedule+0x3c/0xb8 [10150.971031] io_schedule+0x20/0x58 [10150.971037] rq_qos_wait+0x100/0x178 [10150.971041] wbt_wait+0xb4/0xf0 [10150.971045] __rq_qos_throttle+0x38/0x50 [10150.971050] blk_mq_make_request+0x128/0x610 [10150.971056] generic_make_request+0xb4/0x2d8 [10150.971060] submit_bio+0x48/0x218 [10150.971217] vdev_disk_io_start+0x670/0x9f8 [zfs] [10150.971307] zio_vdev_io_start+0xdc/0x2b8 [zfs] [10150.971396] zio_execute+0xac/0x110 [zfs] [10150.971484] vdev_queue_io_done+0x18c/0x2a0 [zfs] [10150.971572] zio_vdev_io_done+0xec/0x220 [zfs] [10150.971660] zio_execute+0xac/0x110 [zfs] [10150.971677] taskq_thread+0x2f8/0x570 [spl] [10150.971684] kthread+0xfc/0x128 [10150.971689] ret_from_fork+0x10/0x1c [10150.971695] INFO: task z_rd_int:2158 blocked for more than 241 seconds. [10150.978427] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10150.985323] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10150.993278] z_rd_int D 0 2158 2 0x00000028 [10150.993285] Call trace: [10150.993293] __switch_to+0x104/0x170 [10150.993300] __schedule+0x30c/0x7c0 [10150.993303] schedule+0x3c/0xb8 [10150.993308] io_schedule+0x20/0x58 [10150.993315] rq_qos_wait+0x100/0x178 [10150.993318] wbt_wait+0xb4/0xf0 [10150.993322] __rq_qos_throttle+0x38/0x50 [10150.993328] blk_mq_make_request+0x128/0x610 [10150.993333] generic_make_request+0xb4/0x2d8 [10150.993337] submit_bio+0x48/0x218 [10150.993438] vdev_disk_io_start+0x670/0x9f8 [zfs] [10150.993545] zio_vdev_io_start+0xdc/0x2b8 [zfs] [10150.993636] zio_nowait+0xd4/0x170 [zfs] [10150.993727] vdev_queue_io_done+0x1ec/0x2a0 [zfs] [10150.993815] zio_vdev_io_done+0xec/0x220 [zfs] [10150.993906] zio_execute+0xac/0x110 [zfs] [10150.993923] taskq_thread+0x2f8/0x570 [spl] [10150.993933] kthread+0xfc/0x128 [10150.993937] ret_from_fork+0x10/0x1c [10150.993944] INFO: task z_wr_iss:2163 blocked for more than 241 seconds. [10151.000674] Tainted: P C OE 5.4.0-1018-raspi #20-Ubuntu [10151.007573] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [10151.015527] z_wr_iss D 0 2163 2 0x00000028 [10151.015533] Call trace: [10151.015541] __switch_to+0x104/0x170 [10151.015545] __schedule+0x30c/0x7c0 [10151.015549] schedule+0x3c/0xb8 [10151.015552] io_schedule+0x20/0x58 [10151.015558] rq_qos_wait+0x100/0x178 [10151.015561] wbt_wait+0xb4/0xf0 [10151.015565] __rq_qos_throttle+0x38/0x50 [10151.015570] blk_mq_make_request+0x128/0x610 [10151.015575] generic_make_request+0xb4/0x2d8 [10151.015579] submit_bio+0x48/0x218 [10151.015677] vdev_disk_io_start+0x670/0x9f8 [zfs] [10151.015765] zio_vdev_io_start+0xdc/0x2b8 [zfs] [10151.015852] zio_nowait+0xd4/0x170 [zfs] [10151.015939] vdev_queue_io+0x23c/0x258 [zfs] [10151.016031] zio_vdev_io_start+0x94/0x2b8 [zfs] [10151.016118] zio_nowait+0xd4/0x170 [zfs] [10151.016205] vdev_mirror_io_start+0xa8/0x1b0 [zfs] [10151.016292] zio_vdev_io_start+0x248/0x2b8 [zfs] [10151.016379] zio_execute+0xac/0x110 [zfs] [10151.016395] taskq_thread+0x2f8/0x570 [spl] [10151.016401] kthread+0xfc/0x128 [10151.016405] ret_from_fork+0x10/0x1c. Interestingly, after changing compression algorithm, it starts to use the swap ZVOL, for the first time, which gives a little more runway, until it fails. Running your ZDB command gives me: tyson@ubuntu:~$ sudo zdb -s Yaesu [sudo] password for tyson: capacity operations bandwidth ---- errors ---- description used avail read write read write read write cksum Yaesu 8.49T 2.39T 2.88K 0 53.5M 0 0 0 0 /dev/disk/by-id/usb-QNAP_TR-004_DISK00_51323032493034393139-0:0-part1 3.50T 132G 668 0 7.14M 0 0 0 0 /dev/disk/by-id/usb-QNAP_TR-004_DISK01_51323032493034393139-0:1-part1 2.29T 1.33T 884 0 24.6M 0 0 0 0 /dev/disk/by-id/usb-QNAP_TR-004_DISK02_51323032493034393139-0:2-part1 1.44T 383G 706 0 9.76M 0 0 0 0 /dev/disk/by-id/usb-QNAP_TR-004_DISK03_51323032493034393139-0:3-part1 1.26T 565G 686 0 12.0M 0 0 0 0 Sadly, I haven't got a drive large enough, to hold all of the data in the pool (around 11TB, deduplicated/compressed, and with snapshots!), to try imaging all of the disks, and dumping it into a new pool, without deduplication enabled. :(