Hi, On Thu, Dec 4, 2008 at 6:11 PM, Simon Riggs <[EMAIL PROTECTED]> wrote: > > On Wed, 2008-12-03 at 14:22 +0900, Koichi Suzuki wrote: > >> > There's clearly a huge gain using prefetch, when we have >> > full_page_writes = off. But that does make me think: Why do we need >> > prefetch at all if we use full page writes? There's nothing to prefetch >> > if we can keep it in cache. >> >> Agreed. This is why I proposed prefetch optional through GUC. >> >> > So I'm wondering if we only need prefetch because we're using lesslog? >> > >> > If we integrated lesslog better into the new replication would we be >> > able to forget about doing the prefetch altogether? >> >> In the case of lesslog, almost all the FPW is replaced with >> corresponding incremental log and recovery takes longer. Prefetch >> dramatically improve this, as you will see in the above result. To >> improve recovery time with FPW=off or FPW=on and lesslog=yes, we need >> prefetch. > > It does sound like it is needed, yes. But if you look at the > architecture of synchronous replication in 8.4 then I don't think it > makes sense any more. It would be very useful for the architecture we > had in 8.3, but that time has gone.
Agreed. I also think that lesslog is for archiving in single node rather than replication between multiple nodes. Of course, it's very useful for the user who doesn't use replication.. etc. > So if we find a way of streaming WAL without FPW then this patch makes > sense, but not until then. So far many people have argued in favour of > using FPW=on, which was the whole point of pg_lesslog. Are we now saying > that we would run the primary with FPW=off? If we always recover a database from a base backup, the primary can run with FPW=off. Since we might need a fresh backup when making the failed server catch up with the current primary, such restriction (always recovery from a backup) might not matter. Regards, -- Fujii Masao NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers