SImple ordinary pool, a couple of levels deep, the sort of path I would "ls" without thinking twice on 11.3 to recover a lost file that's in a previous snapshot.

Except on 12-beta1 it hangs the session.

Having reported one bug ( https://ixsystems.atlassian.net/browse/NAS-107049#icft=NAS-107049) related to zfs that turned out to relate to some kind of bug asserting holds during a send/recv, I thought maybe traversing to a snapshot might also involve holds, so it could be the same or similar bug. So I ran the same dtrace on it in one window, and ran "ls (snapshot dir)" in another window.

RESULT:

The moment I hit return after "ls", the session with "ls" hung, and dtrace started dumping text to the screen.

The attached screenshots show the commands used and outputs in both sessions.

The dtrace log is a bit different from the output of https://ixsystems.atlassian.net/browse/NAS-107049#icft=NAS-107049 , so I don't know if it's the exact same bug. But if not, its a close cousin. The dtrace command used was from that bug report, and a sample from the start of the dtrace log is below:

dtrace -n 'zfs-dbgmsg { print(stringof(arg0)); } set-error { stack(); }' > ~/debug2.txt

7 74436 none:zfs-dbgmsg string [ 'z', 'i', 'o', '.', 'c', ':', '2', '0', '0', '0', ':', 'z', 'i', 'o', '', 'd', 'e', 'a', 'd', 'm', 'a', 'n', '', 'i', 'm', 'p', 'l', '(', ')', ':', ' ', 's', 'l', 'o', 'w', ' ', 'z', 'i', 'o', '[', '3', ']', ':', ' ', 'z', 'i', 'o', '=', '0', 'x', 'f', 'f', 'f', 'f', 'f', '8', '3', 'd', '7', '0', '7', '4', '1', '4', 'b', '8', 'x', ' ', 't', 'i', 'm', 'e', 's', 't', 'a', 'm', 'p', '=', '3', '4', '7', '6', '6', '0', '3', '0', '4', '7', '6', '8', '5', '2', ' ', 'd', 'e', 'l', 't', 'a', '=', '1', '3', '0', '7', '5', '0', '9', '3', '4', '8', '7', '2', '7', '1', ' ', 'q', 'u', 'e', 'u', 'e', 'd', '=', '0', ' ', 'i', 'o', '=', '3', '4', '7', '6', '6', '0', '3', '0', '4', '7', '8', '2', '9', '4', ' ', 'p', 'a', 't', 'h', '=', '/', 'd', 'e', 'v', '/', 'n', 'v', 'd', '1', 'p', '1', ' ', 'l', 'a', 's', 't', '=', '2', '4', '3', '6', '8', '1', '9', '4', '9', '3', '8', '3', '7', '9', ' ', 't', 'y', 'p', 'e', '=', '1', ' ', 'p', 'r', 'i', 'o', 'r', 'i', 't', 'y', '=', '0', ' ', 'f', 'l', 'a', 'g', 's', '=', '0', 'x', '1', '8', '0', '8', '8', '0', ' ', 's', 't', 'a', 'g', 'e', '=', '0', 'x', '1', '0', '0', '0', '0', '0', ' ', 'p', 'i', 'p', 'e', 'l', 'i', 'n', 'e', '=', '0', 'x', '1', 'f', '0', '0', '0', '0', '0', ' ', 'p', 'i', 'p', 'e', 'l', 'i', 'n', 'e', '-', 't', 'r' ]

7 74436 none:zfs-dbgmsg string "zio.c:2035:zio_deadman(): zio_wait waiting for hung I/O to pool 'Main_pool'"

2 74396 none:set-error
openzfs.ko`zfsdev_ioctl_common+0x470
openzfs.ko`zfsdev_ioctl+0x29e
kernel`devfs_ioctl+0xb0
kernel`VOP_IOCTL_APV+0x7b
kernel`vn_ioctl+0x16a
kernel`devfs_ioctl_f+0x1e
kernel`kern_ioctl+0x2b7
kernel`sys_ioctl+0xfa
kernel`amd64_syscall+0x387
kernel`0xffffffff80fa50e0

2 74396 none:set-error
openzfs.ko`vdev_config_generate+0x3ab
openzfs.ko`spa_config_generate+0x2a5
openzfs.ko`spa_open_common+0x214
openzfs.ko`spa_get_stats+0x4e
openzfs.ko`zfs_ioc_pool_stats+0x22
openzfs.ko`zfsdev_ioctl_common+0x470
openzfs.ko`zfsdev_ioctl+0x29e
kernel`devfs_ioctl+0xb0
kernel`VOP_IOCTL_APV+0x7b
kernel`vn_ioctl+0x16a
kernel`devfs_ioctl_f+0x1e
kernel`kern_ioctl+0x2b7
kernel`sys_ioctl+0xfa
kernel`amd64_syscall+0x387
kernel`0xffffffff80fa50e0

2 74396 none:set-error
openzfs.ko`vdev_config_generate+0x3d6
openzfs.ko`spa_config_generate+0x2a5
openzfs.ko`spa_open_common+0x214
openzfs.ko`spa_get_stats+0x4e
openzfs.ko`zfs_ioc_pool_stats+0x22
openzfs.ko`zfsdev_ioctl_common+0x470
openzfs.ko`zfsdev_ioctl+0x29e
kernel`devfs_ioctl+0xb0
kernel`VOP_IOCTL_APV+0x7b
kernel`vn_ioctl+0x16a
kernel`devfs_ioctl_f+0x1e
kernel`kern_ioctl+0x2b7
kernel`sys_ioctl+0xfa
kernel`amd64_syscall+0x387
kernel`0xffffffff80fa50e0

2 74396 none:set-error
openzfs.ko`zfs_ioc_pool_stats+0x36
openzfs.ko`zfsdev_ioctl_common+0x470
openzfs.ko`zfsdev_ioctl+0x29e
kernel`devfs_ioctl+0xb0
kernel`VOP_IOCTL_APV+0x7b
kernel`vn_ioctl+0x16a
kernel`devfs_ioctl_f+0x1e
kernel`kern_ioctl+0x2b7
kernel`sys_ioctl+0xfa
kernel`amd64_syscall+0x387
kernel`0xffffffff80fa50e0

2 74396 none:set-error
openzfs.ko`vdev_config_generate+0x3ab
openzfs.ko`spa_config_generate+0x2a5
openzfs.ko`spa_open_common+0x214
openzfs.ko`spa_get_stats+0x4e
openzfs.ko`zfs_ioc_pool_stats+0x22
openzfs.ko`zfsdev_ioctl_common+0x470
openzfs.ko`zfsdev_ioctl+0x29e
kernel`devfs_ioctl+0xb0
kernel`VOP_IOCTL_APV+0x7b
kernel`vn_ioctl+0x16a
kernel`devfs_ioctl_f+0x1e
kernel`kern_ioctl+0x2b7
kernel`sys_ioctl+0xfa
kernel`amd64_syscall+0x387
kernel`0xffffffff80fa50e0

2 74396 none:set-error
openzfs.ko`vdev_config_generate+0x3d6
openzfs.ko`spa_config_generate+0x2a5
openzfs.ko`spa_open_common+0x214
openzfs.ko`spa_get_stats+0x4e
openzfs.ko`zfs_ioc_pool_stats+0x22
openzfs.ko`zfsdev_ioctl_common+0x470
openzfs.ko`zfsdev_ioctl+0x29e
kernel`devfs_ioctl+0xb0
kernel`VOP_IOCTL_APV+0x7b
kernel`vn_ioctl+0x16a
kernel`devfs_ioctl_f+0x1e
kernel`kern_ioctl+0x2b7
kernel`sys_ioctl+0xfa
kernel`amd64_syscall+0x387
kernel`0xffffffff80fa50e0