Re: Patch for fail-back without fresh backup

On Mon, Oct 21, 2013 at 7:10 PM, Sawada Masahiko <sawada(dot)mshk(at)gmail(dot)com>wrote:

>
>
> I agree with you.
> If writing FPW is not large performance degradation, it is just idea
> that we can use to write FPW in same timing as checksum enabled.
> i.g., if we support new wal_level, the system writes FPW when a simple
> SELECT updates hint bits. but checksum function is disabled.
> Thought?

I wonder if its too much for this purpose. In fact, we just need a way to
know that a block could have been written on the master which the standby
never saw. So even WAL logging just the block id should be good enough for
pg_rewind to be able to detect and later copy that block from the new
master. Having said that, I don't know if there is general advantage of WAL
logging the exact hint bit update operation for other reasons.

Another difference AFAICS is that checksum feature needs the block to be
backed up only after the first time a hint bit is updated after checkpoint.
But for something like pg_rewind to work, we will need to WAL log every
hint bit update on a page. So we would want to keep it as short as possible.

Thanks,
Pavan

--
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee