Re: Online base backup from the hot-standby

From: Steve Singer <ssinger_pg(at)sympatico(dot)ca>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Jun Ishiduka <ishizuka(dot)jun(at)po(dot)ntts(dot)co(dot)jp>, magnus(at)hagander(dot)net, heikki(dot)linnakangas(at)enterprisedb(dot)com, simon(at)2ndquadrant(dot)com, pgsql-hackers(at)postgresql(dot)org, robertmhaas(at)gmail(dot)com, cedric(dot)villemain(dot)debian(at)gmail(dot)com
Subject: Re: Online base backup from the hot-standby
Date: 2012-01-20 04:01:49
Message-ID: BLU0-SMTP7467A0BBFE563ED7A75CDE8E870@phx.gbl
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On 12-01-17 05:38 AM, Fujii Masao wrote:
> On Fri, Jan 13, 2012 at 5:02 PM, Fujii Masao<masao(dot)fujii(at)gmail(dot)com> wrote:
>> The amount of code changes to allow pg_basebackup to make a backup from
>> the standby seems to be small. So I ended up merging that changes and the
>> infrastructure patch. WIP patch attached. But I'd happy to split the patch again
>> if you want.
> Attached is the updated version of the patch. I wrote the limitations of
> standby-only backup in the document and changed the error messages.

Here is my review of this verison of the patch. I think this patch has
been in every CF for 9.2 and I feel it is getting close to being
committed. The only issue of significants is a crash I encountered
while testing, see below.

I am fine with including the pg_basebackup changes in the patch it also
makes testing some of the other changes possible.

The documentation updates you have are good

I don't see any issues looking at the code.

Testing Review

I encountered this on my first replica (the one based on the master). I
am not sure if it is related to this patch, it happened after the
pg_basebackup against the replica finished.

TRAP: FailedAssertion("!(((xid) != ((TransactionId) 0)))", File:
"twophase.c", Line: 1238)
LOG: startup process (PID 12222) was terminated by signal 6: Aborted
LOG: terminating any other active server processes

A little earlier this postmaster had printed.

LOG: restored log file "00000001000000000000001F" from archive
LOG: restored log file "000000010000000000000020" from archive
cp: cannot stat
`/usr/local/pgsql92git/archive/000000010000000000000021': No such file
or directory
LOG: unexpected pageaddr 0/19000000 in log file 0, segment 33, offset 0
cp: cannot stat
`/usr/local/pgsql92git/archive/000000010000000000000021': No such file
or directory

I have NOT been able to replicate this error and I am not sure exactly
what I had done in my testing prior to that point.

In another test run I had

- set full page writes=off and did a checkpoint
- Started the pg_basebackup
- set full_page_writes=on and did a HUP + some database activity that
might have forced a checkpoint.

I got this message from pg_basebackup.
./pg_basebackup -D ../data3 -l foo -h localhost -p 5438
pg_basebackup: could not get WAL end position from server

I point this out because the message is different than the normal "could
not initiate base backup: FATAL: WAL generated with
full_page_writes=off" thatI normally see. We might want to add a
PQerrorMessage(conn)) to pg_basebackup to print the error details.
Since this patch didn't actually change pg_basebackup I don't think your
required to improve the error messages in it. I am just mentioning this
because it came up in testing.

The rest of the tests I did involving changing full_page_writes
with/without checkpoints and sighups and promoting the replica seemed to
work as expected.

> Regards,

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