Re: pg_basebackup blocking all queries with horrible performance - Mailing list pgsql-admin

On Tue, Jun 12, 2012 at 10:49 AM, Fujii Masao <masao.fujii@gmail.com> wrote:
> On Tue, Jun 12, 2012 at 2:37 AM, Lonni J Friedman <netllama@gmail.com> wrote:
>> On Fri, Jun 8, 2012 at 7:29 PM, Fujii Masao <masao.fujii@gmail.com> wrote:
>>> On Sat, Jun 9, 2012 at 4:30 AM, Lonni J Friedman <netllama@gmail.com> wrote:
>>>> On Thu, Jun 7, 2012 at 11:04 PM, Craig Ringer <ringerc@ringerc.id.au> wrote:
>>>>> On 06/08/2012 09:01 AM, Lonni J Friedman wrote:
>>>>>>
>>>>>> On Thu, Jun 7, 2012 at 5:07 PM, Jerry Sievers<gsievers19@comcast.net>
>>>>>>  wrote:
>>>>>>>
>>>>>>> You might try stopping pg_basebackup in place with SIGSTOP and check
>>>>>>>
>>>>>>> if problem goes away.  SIGCONT and you should  start having
>>>>>>> sluggishness again.
>>>>>>>
>>>>>>> If verified, then any sort of throttling mechanism should work.
>>>>>>
>>>>>>
>>>>>> I'm certain that the problem is triggered only when pg_basebackup is
>>>>>> running.  Its very predictable, and goes away as soon as pg_basebackup
>>>>>> finishes running.  What do you mean by a throttling mechanism?
>>>>>
>>>>>
>>>>> Sure, it only happens when pg_basebackup is running. But if you *pause*
>>>>> pg_basebackup, so it's still running but not currently doing work, does the
>>>>> problem go away? Does it come back when you unpause pg_basebackup? That's
>>>>> what Jerry was telling you to try.
>>>>>
>>>>> If the problem goes away when you pause pg_basebackup and comes back when
>>>>> you unpause it, it's probably a system load problem.
>>>>>
>>>>> If it doesn't go away, it's more likely to be a locking issue or something
>>>>> _other_ than simple load.
>>>>>
>>>>> SIGSTOP ("kill -STOP") pauses a process, and SIGCONT ("kill -CONT") resumes
>>>>> it, so on Linux you can use these to try and find out. When you SIGSTOP
>>>>> pg_basebackup then the postgres backend associated with it should block
>>>>> shortly afterwards as its buffers fill up and it can't send more data, so
>>>>> the load should come off the server.
>>>>>
>>>>> A "throttling mechanism" refers to anything that limits the rate or speed of
>>>>> a thing. In this case, what you want to do if your problem is system
>>>>> overload is to limit the speed at which pg_basebackup does its work so other
>>>>> things can still get work done. In other words you want to throttle it.
>>>>> Typical throttling mechanisms include the "ionice" and "renice" commands to
>>>>> change I/O and CPU priority, respectively.
>>>>>
>>>>> Note that you may need to change the priority of the *backend* that
>>>>> pg_basebackup is using, not necessarily the pg_basebackup command its self.
>>>>> I haven't done enough with Pg's replication to know how that works, so
>>>>> someone else will have to fill that bit in.
>>>>
>>>> Thanks for your reply.  I've confirmed that issuing a SIGSTOP does
>>>> eliminate the thrashing, and issuing a SIGCONT resumes the thrash.
>>>>
>>>> I've looked at iostat output both before & during pg_basebackup runs,
>>>> and I'm not seeing any indication that the problem is due to disk IO
>>>> bottlenecks.  The numbers don't vary very much at all between the good
>>>> & bad times.  This is typical when pg_basebackup is running:
>>>> ########
>>>> Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s
>>>> avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
>>>> md0
>>>>                  0.00     0.00   67.76   68.62     4.42     1.46
>>>> 88.34     0.00    0.00    0.00    0.00   0.00   0.00
>>>> ########
>>>>
>>>> and this is when the system is ok:
>>>> ########
>>>> Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s
>>>> avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
>>>> md0
>>>>                  0.00     0.00   68.04   68.56     4.44     1.46
>>>> 88.39     0.00    0.00    0.00    0.00   0.00   0.00
>>>> ########
>>>>
>>>>
>>>> I looked at vmstat output, but nothing is jumping out at me as being
>>>> dramatically different when pg_basebackup is running.  swap in and
>>>> swap out are zero 100% of the time for the good & bad perf cases.  I
>>>> can post example output if someone is interested, or if there's
>>>> something specific that I should be looking at as a potential problem,
>>>> let me know.
>>>
>>> Did you set synchronous_standby_names to '*'? If so, the problem you
>>> encountered can happen.
>>>
>>> When synchronous_standby_names is '*', you cannot control which
>>> standbys take a role of synchronous standby. The standby which you
>>> expect to run as asynchronous one might be synchronous one. So
>>> my guess is that at first one of your three standbys was running as
>>> synchronous standby, and all queries were executed normally. But
>>> when you started pg_basebackup, pg_basebackup unexpectedly
>>> got the role of synchronous standby from another standby. Since
>>> pg_basebackup doesn't send the information about replication
>>> progress back to the master, all queries (more precisely, transaction
>>> commit) got stuck, and kept waiting for the reply from synchronous
>>> standby.
>>>
>>> You can avoid this problem by setting synchronous_standby_names
>>> to the names of your standbys instead of '*'.
>>
>> I don't have synchronous_standby_names set at all.  I'm only doing
>> asynchronous replication.
>
> Hmm... I have no idea about what happened on your environment, for now.
> Could you show me the self-contained test case?

I'm running the following, which gets piped over ssh to a remote
server (at gigabit ethernet speed):
pg_basebackup -v -D - -x -Ft -U postgres

One thing that I've discovered is that if I throttle back the speed of
what is getting piped to the remote server, that directly correlates
to the load on the server.