Online base backup from the hot-standby

Hi

I would like to develop function for 'Online base backup from the
hot-standby' in PostgreSQL 9.2.

Todo : Allow hot file system backups on standby servers
(http://wiki.postgresql.org/wiki/Todo)

[GOAL]
* Make pg_basebackup to execute to the hot-standby server
and acquire online-base-backup .
- pg_basebackup can be executed to only primary server in
PostgreSQL 9.1 .
- But physical-copy(etc) under processing of pg_basebackup
raises the load of primary server .
- Therefore , this function is necessary .

[Problem]
(There is the following problems when hot-standby acquires
online-base-backup like executing pg_basebackup to the primary
server .)
* pg_start_backup() and pg_stop_backup() can't be executed to the
hot-standby server .
- hot-standby can't insert backup-end record to WAL-files and
can't operate CHECKPOINT .
- Because hot-standby can't write anything in WAL-files .
* hot-standby can't send WAL-files to archive server.
- when pg_stop_backup() is executed to the primary server ,
it waits for completing sending wal to archive server ,
but hot-standby can't do it.

[Policy]
(I create with the following Policy .)
* This function doesn't affect primary server .
- I don't adopt the way which "hot-standby requests primary to
execute pg_basebackup" , because I think about many standbys
is connected with a primary .

[Approach]
* When pg_basebackup is executed to the hot-standby server , it
executes RESTARTPOINT instead of CHECKPOINT .
backup_label is made from the RESTARTPOINT's results , and is sent
to the designated backup server using pg_basebackup connection .
* Instead of inserting backup-end record , hot-standby writes
backup-end-position in backup-history-file and sends to the
designated backup server using pg_basebackup connection .
- In 9.1 , startup process knows backup-end-position from only
backup-end record . In addition to its logic, startup process
can know backup-end-position from backup-history-file .
As a result , startup process can recovery certainly
without backup-end record .

[Precondition]
(As a result of the above-mentioned Policy and Approach , there is
the following restrictions .)
* Immediately after backup starting of WAL must contain
full page writes . But the above-mentioned Approach can't satisfy
the restriction according to circumstances . Because
full_page_writes of primary might equal 'off' .
When standby recovery WAL which is removed full page writes by pg_lesslog
, it is the same .
* Because recovery starts from last CHECKPOINT , it becomes long .
* I has not thought new process that become taking the place of
waiting for completing sending wal to archive server , yet.

[Working Step]
STEP1: Make startup process to acquire backup-end-position from
not only backup-end record but also backup-history-file .
* startup process allows to acquire backup-end-position
from backup-history-file .
* When pg_basebackup is executed , backup-history-file is
sent to the designated backup server .

STEP2: Make pg_start_backup() and pg_stop_backup() to be executed
by the hot-standby server.

[Action until The first CommitFest (on June 15)]
I will create a patch to STEP1 .
(The patch will be able to settle a problem of Omnipitr-backup-slave.)
(a problem of Omnipitr-backup-slave :
http://archives.postgresql.org/pgsql-hackers/2011-03/msg01490.php)
* Shedule of creating STEP2 is the next CommitFest (in September 15)

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Jun Ishizuka
NTT Software Corporation
TEL：045-317-7018
E-Mail: ishizuka(dot)jun(at)po(dot)ntts(dot)co(dot)jp
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