Subject: Re: WIP: WAL prefetch (another approach)
To: Tom Lane <tgl@sss.pgh.pa.us>, Thomas Munro <thomas.munro@gmail.com>
Cc: Stephen Frost <sfrost@snowman.net>, Andres Freund <andres@anarazel.de>,
 Jakub Wartak <Jakub.Wartak@tomtom.com>,
 Alvaro Herrera <alvherre@2ndquadrant.com>,
 Tomas Vondra <tomas.vondra@2ndquadrant.com>,
 Dmitry Dolgov <9erthalion6@gmail.com>, David Steele <david@pgmasters.net>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
References: 
 <CA+hUKGJ1=pOiNjSgXYJnjE3OyRtp8tjMRcON256e5EFpzPpgtA@mail.gmail.com>
 <20210210215033.GA27507@tamriel.snowman.net>
 <103ec76d-8bbc-09e1-460c-8785751431b5@enterprisedb.com>
 <20210212044614.5wqjkjbcmbwzever@alap3.anarazel.de>
 <20210213213930.GM27507@tamriel.snowman.net>
 <eed83eaa-91e1-91fe-32d0-ece36c0fa810@enterprisedb.com>
 <20210214231815.GO27507@tamriel.snowman.net>
 <17b3f450-048c-f3a6-7086-259523f33cbc@enterprisedb.com>
 <20210317214331.GF20766@tamriel.snowman.net>
 <28c3352e-5388-6410-aae7-05907079e6c9@enterprisedb.com>
 <CA+hUKGLHTjbBPFX6E1ZCGVyQD30-vWtH=N6EcHEzW13q+jh7FQ@mail.gmail.com>
 <8dd2dcc2-0b6b-a4ab-c1b3-032c0a5ab1c3@enterprisedb.com>
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 <bab127df-c5c0-6e33-69b1-31471d61ff! 62@enterprisedb.com>
 <CA+hUKG+a4wn7B-qBu9SLc1HAbZRSKbp1Ep=rDeveM-UQz84+jg@mail.gmail.com>
 <773932.1619022622@sss.pgh.pa.us>
From: Tomas Vondra <tomas.vondra@enterprisedb.com>
Message-ID: <b971373e-5f05-e691-d83c-c72a4f67167b@enterprisedb.com>
Date: Wed, 21 Apr 2021 22:07:43 +0200
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On 4/21/21 6:30 PM, Tom Lane wrote:
> Thomas Munro <thomas.munro@gmail.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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