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

[Klaus Wenninger]

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 /dev/disk/by-id/dm-name-SBD_1-3P1 is outdated (age: 11)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 /dev/disk/by-id/dm-name-SBD_1-3P2 is outdated (age: 11)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.
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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