Comment 4 for bug 1847628

I did the same test for USB flash memory and it works normally.
try create a swap on zvol, it's definitely not working well..

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jehos@MacBuntu:~$ sudo fdisk -l /dev/sdc
Disk /dev/sdc: 7.46 GiB, 8002732032 bytes, 15630336 sectors
Disk model: Cruzer Blade
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 91BE0613-2C00-8944-BEFF-12CCBF5DE2B8
jehos@MacBuntu:~$ sudo mkswap -f /dev/sdc
Setting up swapspace version 1, size = 7.5 GiB (8002727936 bytes)
no label, UUID=14bb8b5c-ded5-444e-8a62-a21cbc469d08

jehos@MacBuntu:~$ sudo swapon /dev/sdc

jehos@MacBuntu:~$ free -m
              total used free shared buff/cache available
Mem: 7870 3106 3843 257 920 4260
Swap: 7631 0 7631

jehos@MacBuntu:~$ time stress --vm 100 -t 10
stress: info: [6247] dispatching hogs: 0 cpu, 0 io, 100 vm, 0 hdd
stress: info: [6247] successful run completed in 15s

real 0m15.124s
user 0m0.736s
sys 0m3.536s