Hi Alvaro!
On 23/11/18 1:10 μ.μ., Alvaro Herrera wrote:
> On 2018-Nov-14, Rui DeSousa wrote:
>
>>> On Nov 14, 2018, at 3:31 AM, Achilleas Mantzios <achill@matrix.gatewaynet.com> wrote:
>>>
>>> Our sysadms (seasoned linux/network guys : we have been working here
>>> for more than 10 yrs) were absolute in that we run no firewall or
>>> other traffic shaping system between the two hosts. (if we did the
>>> problem would manifest itself earlier). Can you recommend what to
>>> look for exactly regarding both TCP stacks ? The subscriber node is
>>> a clone of the primary. We have :
>>>
>>> # sysctl -a | grep -i keepaliv
>>> net.ipv4.tcp_keepalive_intvl = 75
>>> net.ipv4.tcp_keepalive_probes = 9
>>> net.ipv4.tcp_keepalive_time = 7200
>> Those keep alive settings are linux’s defaults and work out to be 18
>> hours before the abandon connection is dropped. So, the WAL receiver
>> should have corrected itself after that time. For reference, I run
>> terminating abandon session within 15 mins as they take-up valuable
>> database resources and could potentially hold on to locks, snapshots,
>> etc.
> Where does your 18h figure come from? As I understand it, these numbers
> mean "wait 7200 seconds, then send 9 probes 75 seconds apart", kill the
> connection if not reply is obtained. So that works out to about 131
> minutes (modulo fencepost bug). Certainly not 18 hours ...
Thanks, yes it sums up to 2Hrs 11 Mins. The moments after the primary crushed I didn't have the nerves/patience/guts to
waitthat long and actually prove that the subscriber listened happily to a
ghost/stuck connection.
>
> Now ... I have seen Linux kernel code that seemed to me to cause network
> transmission get stuck *in the kernel* without any way out. Now I'm not
> a kernel expert and I don't know if this applies to your case (maybe it
> got fixed already), but it was definitely some process that was stuck
> with "wchan" set to a network kernel call and way beyond TCP keepalives.
It seems we'll have to upgrade our systems/kernels ASAP. Thanks a lot!
>
--
Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt