Re: checkpoint clarifications needed

Adrian Klaver <adrian.klaver@xxxxxxxxxxx> · Mon, 9 Jan 2017 06:26:29 -0800

On 01/09/2017 06:14 AM, Tom DalPozzo wrote:
Hi, I need some clarifications about checkpoints.
Below here a log from my standby server when started and then some parts
of the interested WAL in the master's cluster  obtained by pg_xlogdump.
Just to have an example to talk on.

1) I see: "LOG:  redo starts at 1/F00A7448" . I was expecting a
checkpoint record around 1/F00A7448 but the related checkpoint record is
at lsn: 1/FCBD7510 instead.
I expected that because I read in the doc :"Checkpoints are points in
the sequence of transactions at which it is guaranteed that the heap and
index data files have been updated with ALL information written BEFORE
that checkpoint".
And I interpreted that as "All information written before that
checkpoint RECORD" but now I guess that one thing is a checkpoint point
and one thing is a checkpoint record. Right?

2) I see that a checkpoint position can be right in the middle of a
group of records related to a transaction (in the example, transaction
id 10684). So a checkpoint position is NOT a consistency state point, right?

3) According to doc at 29.5,  in pg_control the position of the last
checkpoint record (not the checkpoint position itself) is saved  right?

4) If I'm right at 2) then, between the checkpoint position (1/F00A7448
) and the checkpoint record position (1/FCBD7510) there must be a point
where the DB is in a consistency state. If not, in case of crash just
after writing the checkpoint record to the WAL and its position to
pg_control, the system would replay from the checkpoint position (known
by  last checkpoint record) without finding a consistency state. Right?

5) How can we define, in terms of log records, a consistency state position?

https://www.postgresql.org/docs/9.5/static/wal-internals.html

"After a checkpoint has been made and the log flushed, the checkpoint's 
position is saved in the file pg_control. Therefore, at the start of 
recovery, the server first reads pg_control and then the checkpoint 
record; then it performs the REDO operation by scanning forward from the 
log position indicated in the checkpoint record. Because the entire 
content of data pages is saved in the log on the first page modification 
after a checkpoint (assuming full_page_writes is not disabled), all 
pages changed since the checkpoint will be restored to a consistent state."

Best regards
Pupillo

STANDBY SERVER LOG
LOG:  redo starts at 1/F00A7448
....
LOG:  consistent recovery state reached at 2/426DF28
LOG:  invalid record length at 2/426DF28: wanted 24, got 0
LOG:  database system is ready to accept read only connections
LOG:  started streaming WAL from primary at 2/4000000 on timeline 1

FROM PG_XLOGDUMP OF MASTER
........
rmgr: Heap        len (rec/tot):     14/  1186, tx:      10684, lsn:
1/F009EEE0, prev 1/F009EEA8, desc: UPDATE off 1 xmax 10684 ; new off 3
xmax 0, blkref #0: rel 1663/16384/16422 blk 20054, blkref #1: rel
1663/16384/16422 blk 19774
rmgr: Btree       len (rec/tot):      2/    64, tx:      10684, lsn:
1/F009F388, prev 1/F009EEE0, desc: INSERT_LEAF off 2, blkref #0: rel
1663/16384/16428 blk 711
rmgr: XLOG        len (rec/tot):      0/  8193, tx:          0, lsn:
1/F009F3C8, prev 1/F009F388, desc: FPI_FOR_HINT , blkref #0: rel
1663/16384/16484 blk 9663 FPW
rmgr: XLOG        len (rec/tot):      0/  8193, tx:          0, lsn:
1/F00A13E8, prev 1/F009F3C8, desc: FPI_FOR_HINT , blkref #0: rel
1663/16384/16484 blk 9664 FPW
rmgr: XLOG        len (rec/tot):      0/  8193, tx:          0, lsn:
1/F00A3408, prev 1/F00A13E8, desc: FPI_FOR_HINT , blkref #0: rel
1663/16384/16484 blk 9665 FPW
rmgr: XLOG        len (rec/tot):      0/  8193, tx:          0, lsn:
1/F00A5428, prev 1/F00A3408, desc: FPI_FOR_HINT , blkref #0: rel
1663/16384/16484 blk 9666 FPW
rmgr: XLOG        len (rec/tot):      0/  8193, tx:          0, lsn:
1/F00A7448, prev 1/F00A5428, desc: FPI_FOR_HINT , blkref #0: rel
1663/16384/16484 blk 9667 FPW
rmgr: XLOG        len (rec/tot):      0/  8193, tx:          0, lsn:
1/F00A9468, prev 1/F00A7448, desc: FPI_FOR_HINT , blkref #0: rel
1663/16384/16484 blk 9668 FPW
rmgr: XLOG        len (rec/tot):      0/  8193, tx:          0, lsn:
1/F00AB488, prev 1/F00A9468, desc: FPI_FOR_HINT , blkref #0: rel
1663/16384/16484 blk 9669 FPW
rmgr: Heap        len (rec/tot):      8/  8063, tx:      10682, lsn:
1/F00AD4A8, prev 1/F00AB488, desc: LOCK off 5: xid 10682: flags 0
LOCK_ONLY EXCL_LOCK , blkref #0: rel 1663/16384/16422 blk 19787 FPW
rmgr: Heap        len (rec/tot):      8/  8063, tx:      10684, lsn:
1/F00AF440, prev 1/F00AD4A8, desc: LOCK off 2: xid 10684: flags 0
LOCK_ONLY EXCL_LOCK , blkref #0: rel 1663/16384/16422 blk 19774 FPW
rmgr: Heap        len (rec/tot):     14/  4657, tx:      10682, lsn:
1/F00B13D8, prev 1/F00AF440, desc: UPDATE off 5 xmax 10682 ; new off 4
xmax 0, blkref #0: rel 1663/16384/16422 blk 20075 FPW, blkref #1: rel
1663/16384/16422 blk 19787
rmgr: Heap        len (rec/tot):     14/  4657, tx:      10684, lsn:
1/F00B2628, prev 1/F00B13D8, desc: UPDATE off 2 xmax 10684 ; new off 4
xmax 0, blkref #0: rel 1663/16384/16422 blk 20054 FPW, blkref #1: rel
1663/16384/16422 blk 19774
rmgr: Btr
.......
rmgr: Heap2       len (rec/tot):      8/    68, tx:          0, lsn:
1/FCBD7448, prev 1/FCBD7400, desc: CLEAN remxid 10903, blkref #0: rel
1663/16384/16422 blk 2001
rmgr: Heap2       len (rec/tot):      8/    66, tx:          0, lsn:
1/FCBD7490, prev 1/FCBD7448, desc: CLEAN remxid 10903, blkref #0: rel
1663/16384/16422 blk 2003
rmgr: Standby     len (rec/tot):     24/    50, tx:          0, lsn:
1/FCBD74D8, prev 1/FCBD7490, desc: RUNNING_XACTS nextXid 10907
latestCompletedXid 10906 oldestRunningXid 10907
rmgr: XLOG        len (rec/tot):     80/   106, tx:          0, lsn:
1/FCBD7510, prev 1/FCBD74D8, desc: CHECKPOINT_ONLINE redo 1/F00A7448;
tli 1; prev tli 1; fpw true; xid 0:10685; oid 24665; multi 1; offset 0;
oldest xid 584 in DB 1; oldest multi 1 in DB 1; oldest/newest commit
timestamp xid: 0/0; oldest running xid 10682; online
rmgr: Heap        len (rec/tot):      3/   164, tx:      10907, lsn:
1/FCBD7580, prev 1/FCBD7510, desc: INSERT off 25, blkref #0: rel
1663/16384/16484 blk 16398
rmgr: Btree       len (rec/tot):      2/    64, tx:      10907, lsn:
1/FCBD7628, prev 1/FCBD7580, desc: INSERT_LEAF off 60, blkref #0: rel
1663/16384/16490 blk 2722
........

--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx

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