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?
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
........
