Re: Online base backup from the hot-standby

> This version of the patch adds a field into pg_controldata that tries to
> store the source of the base backup while in recovery mode.
> I think your ultimate goal with this patch is to be able to take a
> backup of a running hot-standby slave and recover it as another
> instance. This patch seems to provide the ability to have the second
> slave stop recovery at minRecoveryPoint from the control file.
>
>
> My understanding of the procedure you want to get to to take base
> backups off a slave is
>
> 1. execute pg_start_backup('x') on the slave (*)
> 2. take a backup of the data dir
> 3. call pg_stop_backup() on the slave
> 4. Copy the control file on the slave
>
> This patch only addresses the recovery portions.

Yes.

> I don't think the above comment is very clear on what backupserver is.
> Perhaps
>
> /**
> * backupserver is used while postgresql is in recovery mode to
> * store the location of where the backup comes from.
> * When Postgres starts recovery operations
> * it is set to "none". During recovery it is updated to either "master",
> or "slave"
> * When recovery operations finish it is updated back to "none"
> **/

Done.

> Also shouldn't backupServer be the enum type of 'BackupServer' not int?
> Other enums in the structure such as DBState are defined this way.

Now, this is a same as wal_level, not DBState. No?

> Since I can't yet call pg_start_backup or pg_stop_backup() on the slave
> I am calling them on the master.
> (I also did some testing where I didn't put the system into backup
> mode). I admit that I am not sure what to look for as an indication that
> the system isn't recovering to the correct point. In much of my testing
> I was just verifying that the slave started and my data 'looked' okay.

Updated patch as can execute pg_start/stop_backup() on standby server.
One-pass of above steps(from 1. to 4.) is now done on this.
However, there are conditions.
* Master's full_page_write = on.
* On the slave, do not execute stop/promote operation before pg_stop_backup() is executed.
* the result of pg_start_backup() may exceed the result of pg_stop_backup().

> I seem to get this warning in my logs when I start up the instance based
> on the slave backup.
> LOG: 00000: database system was interrupted while in recovery at log
> time 2011-07-08 18:40:20 EDT
> HINT: If this has occurred more than once some data might be corrupted
> and you might need to choose an earlier recovery target
>
> I'm wondering if this warning is a bit misleading to users because it is
> an expected message when starting up an instance based on a slave backup
> (because the slave was already in recovery mode). If I shutdown this
> instance and start it up again I keep getting the warning. My
> understanding of your patch is that there shouldn't be any risk of
> corruption in that case (assuming your patch has no bugs). Can/should we
> be suppressing this message when we detect that we are recovering from a
> slave backup?

This has not been supported yet.
I do not see what state of this message.

Always happens when backup is taken from slave.
What do you think about an approach to add context, "unless take backup from slave"?

> The direction of the patch has changed a bit during this commit fest. I
> think it would be good to provide an update on the rest of the changes
> you plan for this to be a complete useable feature. That would make it
> easier to comment on something you
> missed versus something your planning on dealing with in the next stage.

I see.

I will provide a patch which can exeute pg_start/stop_backup
including to solve above comment and conditions in next stage.
Then please review.

I change this patch status to "Returned with feedback".

Regards.

--------------------------------------------
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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