$ xz -k9 really.big.file.tar (about 2 GB)
I have a 3G /c/swapfile on a rotating HD at priority 0 for swap.
[ 1.0000000] NetBSD 9.99.56 (GENERIC) #0: Tue Apr 14 14:14:02 UTC 2020
[ 1.0000000] mac%S.Culver.Net@localhost:/c/usr/src/sys/arch/evbarm/compile/obj/GENERIC
[ 102791.8683015] panic: sw_reg_strategy: swap to sparse file
[ 102791.8783036] cpu2: Begin traceback...
[ 102791.8783036] 0xbae43d34: netbsd:db_panic+0x14
[ 102791.8783036] 0xbae43d4c: netbsd:vpanic+0x170
[ 102791.8883019] 0xbae43d64: netbsd:snprintf
[ 102791.8883019] 0xbae43ddc: netbsd:swstrategy+0x3ac
[ 102791.8983034] 0xbae43e04: netbsd:bdev_strategy+0xcc
[ 102791.8983034] 0xbae43e34: netbsd:spec_strategy+0xb8
[ 102791.9083030] 0xbae43e64: netbsd:VOP_STRATEGY+0x58
[ 102791.9083030] 0xbae43eac: netbsd:uvm_swap_io+0x138
[ 102791.9183032] 0xbae43ed4: netbsd:swapcluster_flush.part.2+0x9c
[ 102791.9183032] 0xbae43fac: netbsd:uvm_pageout+0x4b4
[ 102791.9283042] cpu2: End traceback...
Stopped in pid 0.76 (system) at netbsd:cpu_Debugger+0x4: bx r14
db{2}> ps /l
PID LID S CPU FLAGS STRUCT LWP * NAME WAIT
10701 1 3 0 80 9970ed00 sshd netio
8247 1 3 1 0 97b9f880 xz biowait
7175 1 3 2 80 92fb8140 pickup kqueue
11156 1 3 0 80 93922180 python3.8 ttyraw
712 1 3 3 80 92f97600 zsh pause
997 1 3 1 80 92e9bb40 sh wait
940 1 3 0 80 92e9b5c0 zsh pause
427 1 3 1 c0 92e9b300 screen select
586 1 3 1 80 92e9b040 screen pause
331 1 3 1 80 918347c0 zsh pause
575 1 3 0 80 92cfbd40 sshd select
509 1 3 1 80 92cfb7c0 sshd poll
294 1 3 0 80 92aec780 zsh pause
160 1 3 0 80 9191c5c0 sh wait
762 1 3 1 80 92aec4c0 zsh pause
889 1 3 1 80 91991600 screen select
792 1 3 2 80 92aec200 screen pause
625 1 3 0 80 91991b80 zsh pause
496 1 3 2 80 919918c0 sshd select
654 1 3 1 80 9296ecc0 sshd poll
643 1 3 2 80 91d7d3c0 zsh ttyraw
231 1 3 3 80 91d7d100 sh wait
676 1 3 1 80 9191c880 zsh pause
793 1 3 1 80 9296e740 screen select
860 1 3 2 80 91991340 screen pause
98 1 3 1 80 9296e480 zsh pause
488 1 3 0 80 928ad700 sshd select
751 1 3 3 80 9296e1c0 sshd poll
529 1 3 0 80 928adc80 zsh ttyraw
777 1 3 0 80 922b2400 sh wait
709 1 3 2 80 928ad9c0 zsh pause
611 1 3 0 80 928ad180 screen select
760 1 3 0 80 9179c780 screen pause
629 1 3 0 80 91d7dc00 zsh pause
716 1 3 3 c0 915eea80 login wait
400 1 3 2 80 928ad440 cron nanoslp
466 1 3 0 80 9191cb40 inetd kqueue
638 1 3 2 80 922b26c0 qmgr kqueue
622 1 3 1 80 922b2980 master kqueue
426 1 3 2 80 91d74900 sshd select
349 1 3 1 80 91d7d680 timed poll
415 1 2 1 1000000 91d740c0 ntpd
372 1 3 0 80 91d74bc0 devpubd devmon
211 1 3 3 80 91991080 syslogd kqueue
187 1 3 0 80 91834d40 mdnsd select
1 1 3 1 80 916a2c40 init wait
0 > 80 7 1 200 91d74640 swapiod
0 71 3 1 200 914fed00 physiod physiod
0 69 3 1 200 916af9c0 bwfm0 bwfm0
0 79 3 0 280 9179c4c0 VCHIQka-0 lnxcmplt
0 78 3 1 200 9179c200 pooldrain pooldrain
0 77 3 0 200 91797cc0 ioflush syncer
0 > 76 7 2 240 91797a00 pgdaemon
0 75 3 3 280 91797740 vcaudio data
0 74 3 3 280 91797480 VCHIQs-0 semacv
0 73 3 3 280 917971c0 VCHIQr-0 semacv
0 72 3 3 280 916afc80 VCHIQ-0 semacv
0 70 3 0 200 916af700 scsibus1 sccomp
0 38 3 0 200 914fe4c0 scsibus0 sccomp
0 68 3 3 200 916af440 usb0 usbevt
0 67 3 1 200 916af180 npfgc-0 npfgccv
0 66 3 0 200 916a2980 rt_free rt_free
0 65 3 0 200 916a26c0 unpgc unpgc
0 64 3 2 200 916a2400 key_timehandler key_timehandler
0 63 3 3 200 916a2140 icmp6_wqinput/3 icmp6_wqinput
0 62 3 2 200 91695c00 icmp6_wqinput/2 icmp6_wqinput
0 61 3 1 200 91695940 icmp6_wqinput/1 icmp6_wqinput
0 60 3 0 200 91695680 icmp6_wqinput/0 icmp6_wqinput
0 59 3 1 200 916953c0 nd6_timer nd6_timer
0 58 3 3 200 91695100 carp6_wqinput/3 carp6_wqinput
0 57 3 2 200 91693bc0 carp6_wqinput/2 carp6_wqinput
0 56 3 1 200 91693900 carp6_wqinput/1 carp6_wqinput
0 55 3 0 200 91693640 carp6_wqinput/0 carp6_wqinput
0 54 3 3 200 91693380 carp_wqinput/3 carp_wqinput
0 53 3 2 200 916930c0 carp_wqinput/2 carp_wqinput
0 52 3 1 200 91686b80 carp_wqinput/1 carp_wqinput
0 51 3 0 200 915ee240 carp_wqinput/0 carp_wqinput
0 45 3 3 200 916868c0 icmp_wqinput/3 icmp_wqinput
0 44 3 2 200 91686600 icmp_wqinput/2 icmp_wqinput
0 43 3 1 200 91686340 icmp_wqinput/1 icmp_wqinput
0 42 3 0 200 91686080 icmp_wqinput/0 icmp_wqinput
0 40 3 2 200 914fea40 rt_timer rt_timer
0 41 3 3 200 915f1b40 vmem_rehash vmem_rehash
0 39 3 1 200 915ee7c0 vcmbox0 vcmbox0
0 50 3 1 200 915f1880 usbtask-dr usbtsk
0 49 3 0 200 915f15c0 usbtask-hc usbtsk
0 48 3 3 200 915f1300 dwc2 dwc2
0 47 3 0 200 915f1040 sdmmc1 mmctaskq
0 46 3 0 200 915eed40 sdmmc0 mmctaskq
0 37 3 3 200 914fe200 xcall/3 xcall
0 36 1 3 200 914fbcc0 softser/3
0 35 1 3 200 914fba00 softclk/3
0 34 1 3 200 914fb740 softbio/3
0 33 1 3 200 914fb480 softnet/3
0 > 32 1 3 201 914fb1c0 idle/3
0 31 3 2 200 914f7c80 xcall/2 xcall
0 30 1 2 200 914f79c0 softser/2
0 29 1 2 200 914f7700 softclk/2
0 28 1 2 200 914f7440 softbio/2
0 27 1 2 200 914f7180 softnet/2
0 26 1 2 201 914f3c40 idle/2
0 25 3 1 200 914f3980 xcall/1 xcall
0 24 1 1 200 914f36c0 softser/1
0 23 1 1 200 914f3400 softclk/1
0 22 1 1 200 914f3140 softbio/1
0 21 1 1 200 914dcc00 softnet/1
0 20 1 1 201 914dc940 idle/1
0 19 3 0 200 914dc680 lnxpwrwq lnxpwrwq
0 18 3 0 200 914dc3c0 lnxlngwq lnxlngwq
0 17 3 0 200 914dc100 lnxsyswq lnxsyswq
0 16 3 0 200 914d8bc0 lnxrcugc lnxrcugc
0 15 3 0 200 914d8900 sysmon smtaskq
0 14 3 3 200 914d8640 pmfsuspend pmfsuspend
0 13 3 2 200 914d8380 pmfevent pmfevent
0 12 3 0 200 914d80c0 sopendfree sopendfr
0 11 3 3 200 914c4b80 iflnkst iflnkst
0 10 3 0 200 914c48c0 nfssilly nfssilly
0 9 3 2 240 914c4600 vdrain vdrain
0 8 3 0 200 914c4340 modunload mod_unld
0 7 3 0 200 914c4080 xcall/0 xcall
0 6 1 0 200 914c1b40 softser/0
0 5 1 0 200 914c1880 softclk/0
0 4 1 0 200 914c15c0 softbio/0
0 3 1 0 200 914c1300 softnet/0
0 > 2 1 0 201 914c1040 idle/0
0 1 3 1 200 807fa940 swapper uvm
db{2}> bt /a
0xbae43d4c: netbsd:vpanic+0x10
0xbae43d64: netbsd:snprintf
0xbae43ddc: netbsd:swstrategy+0x3ac
0xbae43e04: netbsd:bdev_strategy+0xcc
0xbae43e34: netbsd:spec_strategy+0xb8
0xbae43e64: netbsd:VOP_STRATEGY+0x58
0xbae43eac: netbsd:uvm_swap_io+0x138
0xbae43ed4: netbsd:swapcluster_flush.part.2+0x9c
0xbae43fac: netbsd:uvm_pageout+0x4b4
db{2}> set mach cpu 0
Unknown variable
db{2}> mach cpu 0
kdb_trap: switching to cpu0
Stopped in pid 0.2 (system) at netbsd:cpu_idle+0x48: mov r3, #0
; #0x0
db{0}> bt /a
[ 102791.9283042] Non-emulated page fault with intr_depth > 0
[ 102791.9283042] Fatal kernel mode data abort: 'Translation Fault (S)'
[ 102791.9283042] trapframe: 0x80b0c638
[ 102791.9283042] FSR=00000005, FAR=00000080, spsr=20000113
[ 102791.9283042] r0 =00000008, r1 =8008706c, r2 =806a908c, r3 =80642558
[ 102791.9283042] r4 =00000080, r5 =8046efe8, r6 =80a73550, r7 =80b0c980
[ 102791.9283042] r8 =807ef5e4, r9 =0000ffff, r10=00000000, r11=80b0cc44
[ 102791.9283042] r12=00000001, ssp=80b0c688, slr=80082420, pc =8008244c
Faulted in DDB; continuing...
db{0}> mach cpu 1
kdb_trap: switching to cpu1
Stopped in pid 0.80 (system) at netbsd:_kernel_lock+0xb4: cmp r3, #1
; #0x1
db{1}> bt /a
0xbb1ccf04: netbsd:_kernel_lock+0xc
0xbb1ccf34: netbsd:sleepq_block+0x1fc
0xbb1ccf64: netbsd:cv_wait+0x128
0xbb1ccfac: netbsd:workqueue_worker+0xb8
db{1}> mach cpu 2
kdb_trap: switching to cpu2
Stopped in pid 0.76 (system) at netbsd:cpu_Debugger+0x4: bx r14
db{2}> bt /a
0xbae43d4c: netbsd:vpanic+0x10
0xbae43d64: netbsd:snprintf
0xbae43ddc: netbsd:swstrategy+0x3ac
0xbae43e04: netbsd:bdev_strategy+0xcc
0xbae43e34: netbsd:spec_strategy+0xb8
0xbae43e64: netbsd:VOP_STRATEGY+0x58
0xbae43eac: netbsd:uvm_swap_io+0x138
0xbae43ed4: netbsd:swapcluster_flush.part.2+0x9c
0xbae43fac: netbsd:uvm_pageout+0x4b4
db{2}> mach cpu 3
kdb_trap: switching to cpu3
Stopped in pid 0.32 (system) at netbsd:cpu_idle+0x48: mov r3, #0
; #0x0
db{3}> bt /a
0x8062b8cc: 6e72656b
--- tf 0x0 ---