Re: [PATCHES] Doc update for pg_start_backup

On Thu, 2007-06-28 at 23:35 -0400, Tom Lane wrote:
> Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
> > Added a note to the docs that pg_start_backup can take a long time to
> > finish now that we spread out checkpoints:
>
> I was starting to wordsmith this, and then wondered whether it's not
> just a stupid idea for pg_start_backup to act that way. The reason
> you're doing it is to take a base backup, right? What are you going
> to take the base backup with? I do not offhand know of any backup
> tools that don't suck major amounts of I/O bandwidth. That being
> the case, you're simply not going to schedule the operation during
> full-load periods.

Well, that assumes you can predict a time of reduced load and that time
critical activities won't happen at that point. Many times you can, but
I see no reason to force a checkpoint immediate.

If you use snapshots you can copy the data away in your own time, so not
all backup mechanisms draw extensive/high priority I/O power.

> And that leads to the conclusion that
> pg_start_backup should just use CHECKPOINT_IMMEDIATE and not slow
> you down.

I would prefer the default to be do this slowly. If there is a reason to
do it fast, maybe, but we should err towards low impact.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com