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pg_start_backup without checkpoint patch (a part of Synch Rep)

From: "Fujii Masao" <masao(dot)fujii(at)gmail(dot)com>
To: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: pg_start_backup without checkpoint patch (a part of Synch Rep)
Date: 2008-12-27 12:28:52
Message-ID: 3f0b79eb0812270428y1f6f2422ve940caab53d63142@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Hi,

Attached is the self-contained patch to skip checkpoint at pg_start_backup.
This is a part of Synch Rep patches, and was discussed in the following
thread.
http://archives.postgresql.org/message-id/3f0b79eb0812240710j7e613f3atfd6b6fc27403546e@mail.gmail.com

In Synch Rep, we basically have to get a fresh backup, to make the failed
server catch up with the primary after failover. But getting an online-backup
is expensive operation because of pg_start_backup's checkpoint and data
copying. Especially since pg_start_backup is performed as one transaction,
its checkpoint might not only increase I/O traffic but also cause
long-transaction which prevents VACUUM and HOT. This patch makes
pg_start_backup skip a checkpoint if possible, and works out the above
problem.

Specifically, pg_start_backup uses the last checkpoint instead of doing a
new checkpoint if full_page_writes = on since the last checkpoint, which
guarantees that all the full-pages required for PITR are written.

There is one constraint: XLR_BKP_REMOVABLE flag cannot be marked
correctly, because we cannot judge whether the full-pages (generated
before pg_start_backup) are removal or not. AFAIK, this flag is only used
by pglesslog. Should we add a new option to specify whether checkpoint
is skippable, for the backward compatibility?

Regards,

-- 
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

Attachment: bkp_without_ckpt_v3.patch
Description: text/x-patch (25.4 KB)

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