Re: Check that streaming replica received all data after master shutdown

On 01/13/2015 12:11 PM, Vladimir Borodin wrote:
>
> 05 янв. 2015 г., в 18:15, Vladimir Borodin <root@simply.name> написал(а):
>
>> Hi all.
>>
>> I have a simple script for planned switchover of PostgreSQL (9.3 and 9.4) master to one of its replicas. This script
checksa lot of things before doing it and one of them is that all data from master has been received by replica that is
goingto be promoted. Right now the check is done like below: 
>>
>> On the master:
>>
>> postgres@pgtest03d ~ $ psql -t -A -c 'select pg_current_xlog_location();'
>> 0/33000090
>> postgres@pgtest03d ~ $ /usr/pgsql-9.3/bin/pg_ctl stop -m fast
>> waiting for server to shut down.... done
>> server stopped
>> postgres@pgtest03d ~ $ /usr/pgsql-9.3/bin/pg_controldata | head
>> pg_control version number:            937
>> Catalog version number:               201306121
>> Database system identifier:           6061800518091528182
>> Database cluster state:               shut down
>> pg_control last modified:             Mon 05 Jan 2015 06:47:57 PM MSK
>> Latest checkpoint location:           0/34000028
>> Prior checkpoint location:            0/33000028
>> Latest checkpoint's REDO location:    0/34000028
>> Latest checkpoint's REDO WAL file:    0000001B0000000000000034
>> Latest checkpoint's TimeLineID:       27
>> postgres@pgtest03d ~ $
>>
>> On the replica (after shutdown of master):
>>
>> postgres@pgtest03g ~ $ psql -t -A -c "select pg_xlog_location_diff(pg_last_xlog_replay_location(), '0/34000028');"
>> 104
>> postgres@pgtest03g ~ $
>>
>> These 104 bytes seems to be the size of shutdown checkpoint record (as I can understand from pg_xlogdump output).
>>
>> postgres@pgtest03g ~/9.3/data/pg_xlog $ /usr/pgsql-9.3/bin/pg_xlogdump -s 0/33000090 -t 27
>> rmgr: XLOG        len (rec/tot):      0/    32, tx:          0, lsn: 0/33000090, prev 0/33000028, bkp: 0000, desc:
xlogswitch 
>> rmgr: XLOG        len (rec/tot):     72/   104, tx:          0, lsn: 0/34000028, prev 0/33000090, bkp: 0000, desc:
checkpoint:redo 0/34000028; tli 27; prev tli 27; fpw true; xid 0/6010; oid 54128; multi 1; offset 0; oldest xid 1799 in
DB1; oldest multi 1 in DB 1; oldest running xid 0; shutdown 
>> pg_xlogdump: FATAL:  error in WAL record at 0/34000028: record with zero length at 0/34000090
>>
>> postgres@pgtest03g ~/9.3/data/pg_xlog $
>>
>> I’m not sure that these 104 bytes will always be 104 bytes to have a strict equality while checking. Could it change
inthe future? Or is there a better way to understand that streaming replica received all data after master shutdown?
Thecheck that pg_xlog_location_diff returns 104 bytes seems a bit strange. 

Don't rely on it being 104 bytes. It can vary across versions, and
across different architectures.

You could simply check that the standby's pg_last_xlog_replay_location()
 > master's "Latest checkpoint location", and not care about the exact
difference.

- Heikki