Re: directory archive format for pg_dump

On Thursday 16 December 2010 23:34:02 Heikki Linnakangas wrote:
> On 17.12.2010 00:29, Andres Freund wrote:
> > On Thursday 16 December 2010 19:33:10 Joachim Wieland wrote:
> >> On Thu, Dec 16, 2010 at 12:48 PM, Heikki Linnakangas
> >> 
> >> <heikki.linnakangas@enterprisedb.com>  wrote:
> >>> As soon as we have parallel pg_dump, the next big thing is going to be
> >>> parallel dump of the same table using multiple processes. Perhaps we
> >>> should prepare for that in the directory archive format, by allowing
> >>> the data of a single table to be split into multiple files. That way
> >>> parallel pg_dump is simple, you just split the table in chunks of
> >>> roughly the same size, say 10GB each, and launch a process for each
> >>> chunk, writing to a separate file.
> >> 
> >> How exactly would you "just split the table in chunks of roughly the
> >> same size" ? Which queries should pg_dump send to the backend? If it
> >> just sends a bunch of WHERE queries, the server would still scan the
> >> same data several times since each pg_dump client would result in a
> >> seqscan over the full table.
> > 
> > I would suggest implementing<  >  support for tidscans and doing it in
> > segment size...
> 
> I don't think there's any particular gain from matching the server's
> data file segment size, although 1GB does sound like a good chunk size
> for this too.
Its noticeable more efficient reading from different files in different processes 
in comparison to all hammering the same file.

Andres