Message-ID: <e903a39dc91f29d9e3eb3fb8bd50c455a21df0d6.camel@cybertec.at>
Subject: Re: Questions on recovery situations (not urgent)
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: sacrophyte@gmail.com, pgsql-admin@lists.postgresql.org
Date: Wed, 29 May 2024 17:05:54 +0200
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On Wed, 2024-05-29 at 07:27 -0500, Charles Schultz wrote:
> We have been learning more about postgres recovery, and I have come acros=
s a couple
> questions that google has not satisfactorily answered for me.
>=20
> 1. pg_waldump refuses to dump a WAL saying "fatal: WAL segment size must =
be a power
>    of two between 1 MB and 1 GB", even though I have successfully=C2=A0us=
ed the WALs for
>    PITR. What's the magic trick to get around this fatal error? I tried u=
sing a key
>    unwrap function (same as in postgresql.conf) to no avail.

That is unlikely, because WAL segments are always within these limits (and =
have a fixed
size that is a power of two.

Did you compress the files?  If yes, you have to uncompress them before fee=
ding them
to "pg_waldump".

> 2. As I cannot dump the WALs, how do I determine the LSN or XID of a tran=
saction in
>    the past?

That is difficult, even with "pg_waldump".  WAL does not contain SQL statem=
ents.
At best, you can identify which files were modified and what was done with =
them,
so at best you can guess that a table was dropped, because a couple of file=
s get
deleted.  But that could also have been a VACUUM (FULL).

Essentially, you have to guess a good point in time to restore to.

> 3. When restoring forward, I have found=C2=A0recovery_target_time does no=
t work very
>    well (maybe lack of granularity?), and=C2=A0recovery_target_lsn is bet=
ter, but
>   =C2=A0pg_current_wal_lsn() only gives me the LSN before a transaction. =
Is there a way
>    to get the XID of a specific transaction?

"recovery_target_time" works just fine.

The function pg_current_xact_id() gives you the current transaction ID.

If you know ahead of time that you may want to restore to a certain point,
you can use the function pg_create_restore_point() to define a restore poin=
t
to which you can restore using the parameter "recovery_target_name".

> 4. How would I get the OID of a database and a table when the database=C2=
=A0is down?
>    I wish to map the filesystem names to database objects after a crash.
>    Is there a tool/app that can parse pg datafiles? I have to assume that=
 if
>    there is not already one, it is not too hard to write such a tool if o=
ne
>    has experience with PG file headers and reading the opensource=C2=A0co=
de.

I am not aware of such a tool, other than "postgres".
You can start the server and use "oid2name".

Yours,
Laurenz Albe