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

Klaus Wenninger kwenning at redhat.com
Wed Feb 16 10:59:04 EST 2022

On Wed, Feb 16, 2022 at 4:26 PM Klaus Wenninger <kwenning at redhat.com> wrote:

> 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 /dev/disk/by-id/dm-name-SBD_1-3P1 is outdated (age: 11)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 /dev/disk/by-id/dm-name-SBD_1-3P2 is outdated (age: 11)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
> Sry forgotten to address the following.

Guess your sbd-package predates
and thus doesn't properly destroy the io-context using the aio-api.
This flaw has been in kind of since ever and I actually found it due to a
kernel-issue that made
all block-io done the way sbd is doing it (aio + O_SYNC + O_DIRECT Actually
never successfully
tracked it down to the real kernel issue playing with kprobes. But it was
gone on the next kernel
) timeout.
Without survival on pacemaker it would have suicided after
msgwait-timeout (10s in your case probably).
Would be interesting what happens if you raise msgwait-timeout to a value
that would allow
another read attempt.
Does your setup actually recover? Could be possible that it doesn't missing
the fix referenced above.


>> 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.
> 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.
> Regards,
> Klaus
>> Regards,
>> Ulrich
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