Hangs writing to ZFS with fixed buffers #113
Description
When run on a ZFS partition (Ubuntu 22.04.4 LTS (GNU/Linux 5.15.0-102-generic x86_64)), this program spins forever and cannot be killed:
let rec wait_with_retry uring =
match Uring.wait uring with
| None -> wait_with_retry uring (* Interrupted *)
| Some { result; data } -> result, data
let () =
let uring = Uring.create ~queue_depth:2 () in
let buf = Cstruct.of_string "ab" in
Uring.set_fixed_buffer uring buf.buffer |> Result.get_ok;
let fd = Unix.openfile "test.data" [O_CREAT; O_TRUNC; O_RDWR] 0o600 in
for i = 0 to 1 do
let job = Uring.write_fixed uring fd ~file_offset:(Optint.Int63.of_int i) ~off:i ~len:1 () in
assert (Option.is_some job);
let x = Uring.submit uring in
assert (x = 1);
let result, () = wait_with_retry uring in
assert (result = 1);
doneBased on original report by @patricoferris at ocaml-multicore/eio#715 (comment).
pidstat -t 1 shows:
10:40:58 UID TGID TID %usr %system %guest %wait %CPU CPU Command
10:40:59 1000 1027 - 0.00 99.02 0.00 0.00 99.02 0 main.exe
10:40:59 1000 - 1048 0.00 98.04 0.00 0.98 98.04 0 |__iou-wrk-1027
perf record -g shows:
- zpl_iter_write ▒
- 98.63% zfs_write ▒
+ 27.21% dmu_tx_assign ▒
+ 26.45% dmu_tx_commit ▒
+ 19.34% dmu_write_uio_dbuf ▒
+ 11.55% dmu_tx_hold_write_by_dnode ▒
+ 5.20% dmu_tx_create ▒
+ 3.83% dmu_tx_hold_sa ▒
0.82% zfs_clear_setid_bits_if_necessary
The process cannot be killed, even with kill -9.