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Re: WIP: WAL prefetch (another approach) - Mailing list pgsql-hackers

From Tom Lane
Subject Re: WIP: WAL prefetch (another approach)
Date
Msg-id 113605.1650911508@sss.pgh.pa.us
Whole thread Raw
In response to Re: WIP: WAL prefetch (another approach)  (Tom Lane <tgl@sss.pgh.pa.us>)
Responses Re: WIP: WAL prefetch (another approach)  (Thomas Munro <thomas.munro@gmail.com>)
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Oh, one more bit of data: here's an excerpt from pg_waldump output after
the failed test:

rmgr: Btree       len (rec/tot):     72/    72, tx:        727, lsn: 0/01903BC8, prev 0/01903B70, desc: INSERT_LEAF off
111,blkref #0: rel 1663/16384/2673 blk 9 
rmgr: Btree       len (rec/tot):     72/    72, tx:        727, lsn: 0/01903C10, prev 0/01903BC8, desc: INSERT_LEAF off
141,blkref #0: rel 1663/16384/2674 blk 7 
rmgr: Standby     len (rec/tot):     42/    42, tx:        727, lsn: 0/01903C58, prev 0/01903C10, desc: LOCK xid 727 db
16384rel 16391  
rmgr: Transaction len (rec/tot):    437/   437, tx:        727, lsn: 0/01903C88, prev 0/01903C58, desc: COMMIT
2022-04-2520:16:03.374197 CEST; inval msgs: catcache 80 catcache 79 catcache 80 catcache 79 catcache 55 catcache 54
catcache7 catcache 6 catcache 7 catcache 6 catcache 7 catcache 6 catcache 7 catcache 6 catcache 7 catcache 6 catcache 7
catcache6 catcache 7 catcache 6 catcache 7 catcache 6 snapshot 2608 relcache 16391 
rmgr: Heap        len (rec/tot):     59/    59, tx:        728, lsn: 0/01903E40, prev 0/01903C88, desc: INSERT+INIT off
1flags 0x00, blkref #0: rel 1663/16384/16391 blk 0 
rmgr: Heap        len (rec/tot):     59/    59, tx:        728, lsn: 0/01903E80, prev 0/01903E40, desc: INSERT off 2
flags0x00, blkref #0: rel 1663/16384/16391 blk 0 
rmgr: Transaction len (rec/tot):     34/    34, tx:        728, lsn: 0/01903EC0, prev 0/01903E80, desc: COMMIT
2022-04-2520:16:03.379323 CEST 
rmgr: Heap        len (rec/tot):     59/    59, tx:        729, lsn: 0/01903EE8, prev 0/01903EC0, desc: INSERT off 3
flags0x00, blkref #0: rel 1663/16384/16391 blk 0 
rmgr: Heap        len (rec/tot):     59/    59, tx:        729, lsn: 0/01903F28, prev 0/01903EE8, desc: INSERT off 4
flags0x00, blkref #0: rel 1663/16384/16391 blk 0 
rmgr: Transaction len (rec/tot):     34/    34, tx:        729, lsn: 0/01903F68, prev 0/01903F28, desc: COMMIT
2022-04-2520:16:03.381720 CEST 

The error is complaining about not being able to read 0/01903E40,
which AFAICT is from the first "INSERT INTO sample_tbl" command,
which most certainly ought to be down to disk at this point.

Also, I modified the test script to see what WAL LSNs it thought
it was dealing with, and got

+\echo 'wal_lsn1 = ' :wal_lsn1
+wal_lsn1 =  0/1903E40
+\echo 'wal_lsn2 = ' :wal_lsn2
+wal_lsn2 =  0/1903EE8

confirming that idea of where 0/01903E40 is in the WAL history.
So this is sure looking like a bug somewhere in xlogreader.c,
not in pg_walinspect.

            regards, tom lane

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