Re: [HACKERS] Re: 9.4.1 -> 9.4.2 problem: could not access status of transaction 1

Alvaro Herrera wrote:

> Anyway here's a quick script to almost-reproduce the problem.

Meh. Really attached now.

I also wanted to post the error messages we got:

2015-05-27 16:15:17 UTC [4782]: [3-1] user=,db= LOG: entering standby mode
2015-05-27 16:15:18 UTC [4782]: [4-1] user=,db= LOG: restored log file "00000001000073DD000000AD" from archive
2015-05-27 16:15:18 UTC [4782]: [5-1] user=,db= FATAL: could not access status of transaction 4624559
2015-05-27 16:15:18 UTC [4782]: [6-1] user=,db= DETAIL: Could not read from file "pg_multixact/offsets/0046" at offset 147456: Success.
2015-05-27 16:15:18 UTC [4778]: [4-1] user=,db= LOG: startup process (PID 4782) exited with exit code 1
2015-05-27 16:15:18 UTC [4778]: [5-1] user=,db= LOG: aborting startup due to startup process failure

We pg_xlogdumped the offending segment and see that there is a
checkpoint record with oldestMulti=4624559. Curiously, there are no
other records with rmgr=MultiXact in that segment.

Note that the file exists (otherwise the error would say "could not
open" rather than "could not read"); this is also why errno says
"Success" rather than some actual error; this is the code:

errno = 0;
if (read(fd, shared->page_buffer[slotno], BLCKSZ) != BLCKSZ)
{
slru_errcause = SLRU_READ_FAILED;
slru_errno = errno;
CloseTransientFile(fd);
return false;
}

My guess is that the file existed, and perhaps had one or more pages,
but the wanted page doesn't exist, so we tried to read but got 0 bytes
back. read() returns 0 in this case but doesn't set errno.

I didn't find a way to set things so that the file exists but is of
shorter contents than oldestMulti by the time the checkpoint record is
replayed.

--
Álvaro Herrera http://www.2ndQuadrant.com/
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