Re: WIP: WAL prefetch (another approach)

On 4/21/21 6:30 PM, Tom Lane wrote:
> Thomas Munro <thomas(dot)munro(at)gmail(dot)com> writes:
>> Yeah, it would have been nice to include that but it'll have to be for
>> v15 due to lack of time to convince myself that it was correct. I do
>> intend to look into more concurrency of that kind for v15. I have
>> pushed these patches, updated to be disabled by default.
>
> I have a fairly bad feeling about these patches. I've already fixed
> one critical bug (see 9e4114822), but I am still seeing random, hard
> to reproduce failures in WAL replay testing. It looks like sometimes
> the "decoded" version of a WAL record doesn't match what I see in
> the on-disk data, which I'm having no luck tracing down.
>
> Another interesting failure I just came across is
>
> 2021-04-21 11:32:14.280 EDT [14606] LOG: incorrect resource manager data checksum in record at F/438000A4
> TRAP: FailedAssertion("state->decoding", File: "xlogreader.c", Line: 845, PID: 14606)
> 2021-04-21 11:38:23.066 EDT [14603] LOG: startup process (PID 14606) was terminated by signal 6: Abort trap
>
> with stack trace
>
> #0 0x90b669f0 in kill ()
> #1 0x90c01bfc in abort ()
> #2 0x0057a6a0 in ExceptionalCondition (conditionName=<value temporarily unavailable, due to optimizations>, errorType=<value temporarily unavailable, due to optimizations>, fileName=<value temporarily unavailable, due to optimizations>, lineNumber=<value temporarily unavailable, due to optimizations>) at assert.c:69
> #3 0x000f5cf4 in XLogDecodeOneRecord (state=0x1000640, allow_oversized=1 '\001') at xlogreader.c:845
> #4 0x000f682c in XLogNextRecord (state=0x1000640, record=0xbfffba38, errormsg=0xbfffba9c) at xlogreader.c:466
> #5 0x000f695c in XLogReadRecord (state=<value temporarily unavailable, due to optimizations>, record=0xbfffba98, errormsg=<value temporarily unavailable, due to optimizations>) at xlogreader.c:352
> #6 0x000e61a0 in ReadRecord (xlogreader=0x1000640, emode=15, fetching_ckpt=0 '\0') at xlog.c:4398
> #7 0x000ea320 in StartupXLOG () at xlog.c:7567
> #8 0x00362218 in StartupProcessMain () at startup.c:244
> #9 0x000fc170 in AuxiliaryProcessMain (argc=<value temporarily unavailable, due to optimizations>, argv=<value temporarily unavailable, due to optimizations>) at bootstrap.c:447
> #10 0x0035c740 in StartChildProcess (type=StartupProcess) at postmaster.c:5439
> #11 0x00360f4c in PostmasterMain (argc=5, argv=0xa006a0) at postmaster.c:1406
> #12 0x0029737c in main (argc=<value temporarily unavailable, due to optimizations>, argv=<value temporarily unavailable, due to optimizations>) at main.c:209
>
>
> I am not sure whether the checksum failure itself is real or a variant
> of the seeming bad-reconstruction problem, but what I'm on about right
> at this moment is that the error handling logic for this case seems
> quite broken. Why is a checksum failure only worthy of a LOG message?
> Why is ValidXLogRecord() issuing a log message for itself, rather than
> being tied into the report_invalid_record() mechanism? Why are we
> evidently still trying to decode records afterwards?
>

Yeah, that seems suspicious.

> In general, I'm not too pleased with the apparent attitude in this
> thread that it's okay to push a patch that only mostly works on the
> last day of the dev cycle and plan to stabilize it later.
>

Was there such attitude? I don't think people were arguing for pushing a
patch's not working correctly. The discussion was mostly about getting
it committed even and leaving some optimizations for v15.

regards

--
Tomas Vondra
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