Re: 12.3 replicas falling over during WAL redo

On 2020-Aug-03, Ben Chobot wrote:

> Alvaro Herrera wrote on 8/3/20 12:34 PM:
> > On 2020-Aug-03, Ben Chobot wrote:
> >
> > Yep. Looking at the ones in block 6501,
> >
> > > rmgr: Btree       len (rec/tot):     72/    72, tx:   76393394, lsn:
> > > A0A/AB2C43D0, prev A0A/AB2C4378, desc: INSERT_LEAF off 41, blkref #0: rel
> > > 16605/16613/60529051 blk 6501
> > > rmgr: Btree       len (rec/tot):     72/    72, tx:   76396065, lsn:
> > > A0A/AC4204A0, prev A0A/AC420450, desc: INSERT_LEAF off 48, blkref #0: rel
> > > 16605/16613/60529051 blk 6501
> > My question was whether the block has received the update that added the
> > item in offset 41; that is, is the LSN in the crashed copy of the page
> > equal to A0A/AB2C43D0? If it's an older value, then the write above was
> > lost for some reason.
>
> How do I tell?

You can use pageinspect's page_header() function to obtain the page's
LSN. You can use dd to obtain the page from the file,

dd if=16605/16613/60529051 bs=8192 count=1 seek=6501 of=/tmp/page.6501

then put that binary file in a bytea column, perhaps like

create table page (raw bytea);
insert into page select pg_read_binary_file('/tmp/page');

and with that you can run page_header:

create extension pageinspect;
select h.* from page, page_header(raw) h;

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