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Re: XLogArchivingActive

From: Andreas Pflug <pgadmin(at)pse-consulting(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: XLogArchivingActive
Date: 2006-05-26 14:17:08
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Tom Lane wrote:
> Andreas Pflug <pgadmin(at)pse-consulting(dot)de> writes:
>>Tom Lane wrote:
>>>I'm not sure you can expect that to work.  The system is not built to
>>>guarantee instantaneous response to mode changes like that.
>>Um, as long as xlog writing stops immediate recycling when 
>>pg_start_backup is executed everything should be fine, since archived 
>>logs are not expected to be present until pg_stop_backup is done.
> Wrong.  You forgot about all the *other* behaviors that change depending
> on XLogArchivingActive, like whether CREATE INDEX gets archived or
> just fsync'd.  I don't think it makes sense for CREATE INDEX to change
> that behavior in midstream, even assuming that it noticed the flag
> change instantly.

Ok, but how can I recognize whether all running commands have safely 
switched to "archiving mode" after enabling it, to continue backing up?

Thought a little about your proposal to use a non-copying 
archive_command, since I only want to have a backup of the state the 
cluster had when backup started, but this won't work because all write 
actions that are not appending (truncate, drop) would remove files 
needed for pre-backup state while possibly not backed up yet, thus the 
WAL archive is needed.
Following your proposal, I could redirect archiving to /dev/null while 
not backing up, but how can I make sure that WAL files of transactions, 
open when starting the backup procedure, are written to the wal 
directory, not lost previously? When pg_start_backup() is executed, I'd 
need the archiver to write all "hot" xlog files again.


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