[ClusterLabs] Antw: [EXT] Re: Q: sbd: Which parameter controls "error: servant_md: slot read failed in servant."?

[Ulrich Windl]

>>> Klaus Wenninger <kwenning at redhat.com> schrieb am 16.02.2022 um 16:26 in
Nachricht
<CALrDAo28tNtQZORawC3Qg1+6pN8_EFbm-MSNrQcLcfa3kPBmqg at mail.gmail.com>:
> On Wed, Feb 16, 2022 at 3:09 PM Ulrich Windl <
> Ulrich.Windl at rz.uni-regensburg.de> wrote:
> 
>> Hi!
>>
>> When changing some FC cables I noticed that sbd complained 2 seconds after
>> the connection went down (event though the device is multi-pathed with
>> other paths being still up).
>> I don't know any sbd parameter being set so low that after 2 seconds sbd
>> would panic. Which parameter (if any) is responsible for that?
>>
>> In fact multipath takes up to 5 seconds to adjust paths.
>>
>> Here are some sample events (sbd-1.5.0+20210720.f4ca41f-3.6.1.x86_64 from
>> SLES15 SP3):
>> Feb 14 13:01:36 h18 kernel: qla2xxx [0000:41:00.0]-500b:3: LOOP DOWN
>> detected (2 7 0 0).
>> Feb 14 13:01:38 h18 sbd[6621]: /dev/disk/by-id/dm-name-SBD_1-3P2:
>> error: servant_md: slot read failed in servant.
>> Feb 14 13:01:38 h18 sbd[6619]: /dev/disk/by-id/dm-name-SBD_1-3P1:
>> error: servant_md: mbox read failed in servant.
>> Feb 14 13:01:40 h18 sbd[6615]:  warning: inquisitor_child: Servant
>> /dev/disk/by-id/dm-name-SBD_1-3P1 is outdated (age: 11)
>> Feb 14 13:01:40 h18 sbd[6615]:  warning: inquisitor_child: Servant
>> /dev/disk/by-id/dm-name-SBD_1-3P2 is outdated (age: 11)
>> Feb 14 13:01:40 h18 sbd[6615]:  warning: inquisitor_child: Majority of
>> devices lost - surviving on pacemaker
>> Feb 14 13:01:42 h18 kernel: sd 3:0:3:2: rejecting I/O to offline device
>> Feb 14 13:01:42 h18 kernel: blk_update_request: I/O error, dev sdbt,
>> sector 2048 op 0x0:(READ) flags 0x4200 phys_seg 1 prio class 1
>> Feb 14 13:01:42 h18 kernel: device-mapper: multipath: 254:17: Failing path
>> 68:112.
>> Feb 14 13:01:42 h18 kernel: sd 3:0:1:2: rejecting I/O to offline device
>>
>> Most puzzling is the fact that sbd reports a problem 4 seconds before the
>> kernel reports an I/O error. I guess sbd "times out" the pending read.
>>
> Yep - that is timeout_io defaulting to 3s.
> You can set it with -I daemon start parameter.
> Together with the rest of the default-timeout-scheme the 3s do make sense.
> Not sure but if you increase that significantly you might have to adapt
> other timeouts.

We extended the timeouts so that sbd would survive an online firmware update of the storage system which may cause it not to respond for up to 30 seconds when the controllers restart.

> There are a certain number of checks regarding relationship of timeouts but
> they might not be exhaustive.
> 
>>
>> The thing is: Both SBD disks are on different storage systems, each being
>> connected by two separate FC fabrics, but still when disconnecting one
>> cable from the host sbd panics.
>> My guess is if "surviving on pacemaker" would not have happened, the node
>> would be fenced; is that right?
>>
>> The other thing I wonder is the "outdated age":
>> How can the age be 11 (seconds) when the disk was disconnected 4 seconds
>> ago?
>> It seems here the age is "current time - time_of_last read" instead of
>> "current_time - time_when read_attempt_started".
>>
> Exactly! And that is the correct way to do it as we need to record the time
> passed since last successful read.
> There is no value in starting the clock when we start the read attempt as
> these attempts are not synced throughout
> the cluster.

I don't understand: There is no heartbeat written to SBD that has to be read; instead the device is polled for messages.
So the important point is how much the polling is delayed by some problem (not by some deliberate sleep).
And I don't see why the measurement has to be synced throughout the cluster: Local is enough.

Well, maybe I'm a bit to picky, but I wrote some code that handles read stalls, and it took me a few iterations to get it right.
The interesting part is when the delay exceeds the polling interval, so critical alerts can be created while waiting for a response, but still counting as just "one" event:
20220215 201347 (W)monitor_thread: no I/O for seq 14 at 503949.8447422 since 8.1015658, exceeding deadline at 503956.0947422 by 1.8515658
20220215 201347 (N)check_thresholds: CRITICAL (max:1): 8.1015658 > 1.0000000
20220215 201349 (W)monitor_thread: no I/O for seq 14 at 503949.8447422 since 10.6018244, exceeding deadline at 503956.0947422 by 4.3518244
20220215 201349 (N)check_thresholds: CRITICAL (max:1): 10.6018244 > 1.0000000
20220215 201352 (W)monitor_thread: no I/O for seq 14 at 503949.8447422 since 13.1020604, exceeding deadline at 503956.0947422 by 6.8520604
20220215 201352 (N)check_thresholds: CRITICAL (max:1): 13.1020604 > 1.0000000
20220215 201354 (W)monitor_thread: no I/O for seq 14 at 503949.8447422 since 15.6022895, exceeding deadline at 503956.0947422 by 9.3522895
20220215 201354 (N)check_thresholds: CRITICAL (max:1): 15.6022895 > 1.0000000
20220215 201357 (W)monitor_thread: no I/O for seq 14 at 503949.8447422 since 18.1025207, exceeding deadline at 503956.0947422 by 11.8525207
20220215 201357 (N)check_thresholds: CRITICAL (max:1): 18.1025207 > 1.0000000
20220215 201359 (W)monitor_thread: no I/O for seq 14 at 503949.8447422 since 20.6027195, exceeding deadline at 503956.0947422 by 14.3527195
20220215 201359 (N)check_thresholds: CRITICAL (max:1): 20.6027195 > 1.0000000
20220215 201400 (W)timing_thread: I/O delay 21.2324845 exceeded poll delay 5.0000000 (seq 14)
20220215 201402 (N)check_thresholds: CRITICAL (max:2): 21.2324845 > 1.0000000

Regards,
Ulrich
> 
> Regards,
> Klaus
> 
>>
>> Regards,
>> Ulrich
>>
>>
>>
>>
>> _______________________________________________
>> Manage your subscription:
>> https://lists.clusterlabs.org/mailman/listinfo/users 
>>
>> ClusterLabs home: https://www.clusterlabs.org/ 
>>
>>