RE: PostgreSQL debug log doesn't record whole procedure(from receiving request to sending response)

"Tom Lane" <tgl@sss.pgh.pa.us>  writes: 
> We can see from this that the server spent 10 seconds in CommitTransaction, so the question is what took so long.
I'd wonder first about end-of-transaction triggers (have you got foreign keys on that table?  maybe an event trigger?),
andthen second about delays in writing/fsyncing WAL (what's the underlying storage?  do you have synchronous_commit
replicationturned on?).
 

We have the same as your second suspicion. Delays in fsyncing WAL as the backend ceph cluster storage and we see there
wassignificant latency increased during that time. Yes. the synchronous_commit is 
 
on. 

fsync                                                   | on                                           | Forces
synchronizationof updates to disk.
 
synchronous_commit                     | on                                           | Sets the current transaction's
synchronizationlevel.
 


> [ shrug... ] Sure, we could put an elog(DEBUG) after every line of code in the server, and then high-level debugging
logswould be even more impossibly voluminous than they are now.  I'd say the existing logging gave you plenty of clue
whereto look.
 

Actually, do not need to write log for every line code. The last debug log we can see is for CommitTransaction which
stateis INPROGRESS. But we can't see when  CommitTransaction state is DONE/COMPLETE from the debug log which I think
thiskind of log is valuable which can help to identify where the delay comes, from server or client from the postgres
serverdebug log only shall be enough without to monitoring and analyzing the TCP message.
 

[2701833-618d1b70.293a09-173273] 2021-11-13 22:25:58.051 GMT <169.254.0.21 UPDATE> DEBUG:  00000: CommitTransaction(1)
name:unnamed; blockState: STARTED; state: INPROGRESS, xid/subid/cid: 23280/1/1
 
[2701833-618d1b70.293a09-173274] 2021-11-13 22:25:58.051 GMT <169.254.0.21 UPDATE> LOCATION:  ShowTransactionStateRec,
xact.c:5333



Best Regards
It always takes longer than you expect, 
even when you take into account
----------------------------------------------------------------------------------------------
Yan-Jack Chen (陈雁)
Tel: +8613957141340
Addr: No.567 XinCheng Rd, Binjiang District, Hangzhou, China, 310053

-----Original Message-----
From: Tom Lane <tgl@sss.pgh.pa.us> 
Sent: 2021年11月15日 23:09
To: Chen, Yan-Jack (NSB - CN/Hangzhou) <yan-jack.chen@nokia-sbell.com>
Cc: pgsql-general@postgresql.org
Subject: Re: PostgreSQL debug log doesn't record whole procedure(from receiving request to sending response)

"Chen, Yan-Jack (NSB - CN/Hangzhou)" <yan-jack.chen@nokia-sbell.com> writes:
>   We recently encounter one issue about PostgreSQL ODBC client doesn¡¯t receive response from PostgreSQL server in
time(client set 5 seconds timeout) occasionally (1 or 2 times per 24 hours).  Both PostgreSQL and its client are
deployedin VM against. It took us days to debug where cause the timeout. We enable PostgreSQL server debug log via
belowconfiguration.
 

We can see from this that the server spent 10 seconds in CommitTransaction, so the question is what took so long.
I'd wonder first about end-of-transaction triggers (have you got foreign keys on that table?  maybe an event trigger?),
andthen second about delays in writing/fsyncing WAL (what's the underlying storage?  do you have synchronous_commit
replicationturned on?).
 

> This mail is to ask why PostgreSQL debug log doesn¡¯t really include the response message delay which may cause
misleadingwhy troubleshooting. It looks to me the debug log doesn¡¯t record the whole procedure. If there are some
developeroptions include the missing part but we didn¡¯t enable?
 

[ shrug... ] Sure, we could put an elog(DEBUG) after every line of code in the server, and then high-level debugging
logswould be even more impossibly voluminous than they are now.  I'd say the existing logging gave you plenty of clue
whereto look.
 

            regards, tom lane