Re: directory archive format for pg_dump

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(dot)linnakangas(at)enterprisedb(dot)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.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com