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Re: pg_walinspect - a new extension to get raw WAL data and WAL stats - Mailing list pgsql-hackers

From	RKN Sai Krishna
Subject	Re: pg_walinspect - a new extension to get raw WAL data and WAL stats
Date	March 25, 2022 18:07:31
Msg-id	CAMVpbFOzzgQMnz2XedRQSx86QrCkD6KW0Rk_fNsKmSG9SbFpxg@mail.gmail.com Whole thread Raw
In response to	Re: pg_walinspect - a new extension to get raw WAL data and WAL stats (Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>)
Responses	Re: pg_walinspect - a new extension to get raw WAL data and WAL stats (Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>)
List	pgsql-hackers

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Hi Bharath,

First look at the patch, bear with me if any of the following comments are repeated.

1. With pg_get_wal_record(lsn), say a WAL record start, end lsn range contains the specified LSN, wouldn't it be more meaningful to show the corresponding WAL record.

For example, upon providing '0/17335E7' as input, and I see get the WAL record ('0/1733618', '0/173409F') as output and not the one with start and end lsn as ('0/17335E0', '0/1733617').

With pg_walfile_name(lsn), we can find the WAL segment file name that contains the specified LSN.

2. I see the following output for pg_get_wal_record. Need to have a look at the spaces I suppose.

rkn=# select * from pg_get_wal_record('0/4041728');
start_lsn | end_lsn | prev_lsn | record_length |

record

-----------+-----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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0/4041728 | 0/40421AF | 0/40416F0 | 2670 | \x6e0a0000d2020000f016040400000000000a0000fef802b400007f7f00000000408fe738a25500000300000000000000010000007f06000000
4000003b0a00000000000012000000100101007f7f7f7f0885e738a25500003000c815380a03000885e738a255000000007f7f7f7f7f7f0000000078674301296000003000f8150020042000000000709e1203909
dba01c89c8601609cd00030986008f8956804d202000000000000000000000000120006001f00030820ffff5f04000000000001400000010000000000000004000000000080bf0200030000000000000000006100
0000610000000000000000000000000000000000000000000000000000000000000000000000330100000000000000bc02000001000000010000000000803f00000000000000b0060000010000000000000017000

3. Should these functions be running in standby mode too? We do not allow WAL control functions to be executed during recovery right?

4. If the wal segment corresponding to the start lsn is removed, but there are WAL records which could be read in the specified input lsn range, would it be better to output the existing WAL records displaying a message that it is a partial list of WAL records and the WAL files corresponding to the rest are already removed, rather than erroring out saying "requested WAL segment has already been removed"?

5. Following are very minor comments in the code

Correct the function description by removing "return the LSN up to which the server has WAL" for IsFutureLSN
In GetXLogRecordInfo, good to have pfree in place for rec_desc, rec_blk_ref, data
In GetXLogRecordInfo, can avoid calling XLogRecGetInfo(record) multiple times by capturing in a variable
In GetWALDetailsGuts, setting end_lsn could be done in single if else and similarly we can club the if statements verifying if the start lsn is a future lsn.

Thanks,

RKN

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Re: pg_walinspect - a new extension to get raw WAL data and WAL stats - Mailing list pgsql-hackers

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