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help / color / mirror / Atom feedFrom: Bharath Rupireddy <[email protected]>
To: Kyotaro Horiguchi <[email protected]>
Cc: Ashutosh Sharma <[email protected]>
Cc: Stephen Frost <[email protected]>
Cc: Jeff Davis <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Andrew Dunstan <[email protected]>
Cc: Greg Stark <[email protected]>
Cc: Jeremy Schneider <[email protected]>
Cc: Bruce Momjian <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: SATYANARAYANA NARLAPURAM <[email protected]>
Cc: [email protected]
Cc: [email protected]
Subject: Re: pg_walinspect - a new extension to get raw WAL data and WAL stats
Date: Thu, 17 Mar 2022 13:53:55 +0530
Message-ID: <CALj2ACVBST5Us6-eDz4q_Gem3rUHSC7AYNOB7tmp9Yqq6PHsXw@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <CALj2ACW3cT5NPrBWzdmXgjw59M4xLL7jcL8C3xu9W3i7wdOyaw@mail.gmail.com>
<[email protected]>
<CAE9k0PmFePyata6MDom_5Fp10gD2pa-yguLGfF6fZx+c9Jzz+w@mail.gmail.com>
<[email protected]>
On Thu, Mar 17, 2022 at 10:48 AM Kyotaro Horiguchi
<[email protected]> wrote:
>
> It still suggests unspecifiable end-LSN..
>
> > select * from pg_get_wal_records_info('4/4B28EB68', '4/4C000060');
> > ERROR: cannot accept future end LSN
> > DETAIL: Last known WAL LSN on the database system is 4/4C000060.
Thanks Kyotaro-san. We can change the detail message to show (current
flush lsn/last replayed lsn - 1), that's what I've done in v11 posted
upthread at [1]. The problem is that all the pg_walinspect functions
would wait for the first valid record in read_local_xlog_page() via
InitXLogReaderState()->XLogFindNextRecord(), see[2].
We have two things to do:
1) Just document the behaviour "pg_walinspect functions will wait for
the first valid WAL record if there is none found after the specified
input LSN/start LSN.". This seems easier but some may see it as a
problem.
2) Have read_local_xlog_page_2 which doesn't wait for future WAL LSN
unlike read_local_xlog_page and like pg_waldump's WALDumpReadPage. It
requires a new function read_local_xlog_page_2 that almost looks like
read_local_xlog_page except wait (pg_usleep) loop, we can avoid code
duplication by moving the read_local_xlog_page code to a static
function read_local_xlog_page_guts(existing params, bool wait):
read_local_xlog_page(params)
read_local_xlog_page_guts(existing params, false);
read_local_xlog_page_2(params)
read_local_xlog_page_guts(existing params, true);
read_local_xlog_page_guts:
if (wait) wait for future wal; ---> existing pg_usleep code in
read_local_xlog_page.
else return;
I'm fine either way, please let me know your thoughts on this?
[1] https://www.postgresql.org/message-id/CALj2ACU8XjbYbMwh5x6hEUJdpRoG9%3DPO52_tuOSf1%3DMO7WtsmQ%40mail...
[2]
postgres=# select pg_current_wal_flush_lsn();
pg_current_wal_flush_lsn
--------------------------
0/1624430
(1 row)
postgres=# select * from pg_get_wal_record_info('0/1624430');
ERROR: cannot accept future input LSN
DETAIL: Last known WAL LSN on the database system is 0/162442F.
postgres=# select * from pg_get_wal_record_info('0/162442f'); --->
waits for the first valid record in read_local_xlog_page.
Regards,
Bharath Rupireddy.
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Subject: Re: pg_walinspect - a new extension to get raw WAL data and WAL stats
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