OK so from the attachments.. cat /proc/<pid>/stack for md1_raid6 [<0>] rq_qos_wait+0xfa/0x170 [<0>] wbt_wait+0x98/0xe0 [<0>] __rq_qos_throttle+0x23/0x30 [<0>] blk_mq_make_request+0x12a/0x5d0 [<0>] generic_make_request+0xcf/0x310 [<0>] submit_bio+0x42/0x1c0 [<0>] md_update_sb.part.71+0x3c0/0x8f0 [md_mod] [<0>] r5l_do_reclaim+0x32a/0x3b0 [raid456] [<0>] md_thread+0x94/0x150 [md_mod] [<0>] kthread+0x112/0x130 [<0>] ret_from_fork+0x22/0x40 Btrfs snapshot flushing might instigate the problem but it seems to me there's some kind of contention or blocking happening within md, and that's why everything stalls. But I can't tell why. Do you have any iostat output at the time of this problem? I'm wondering if md is waiting on disks. If not, try `iostat -dxm 5` and share a few minutes before and after the freeze/hang. -- Chris Murphy
