Re: [FEATURE REQUEST] Streaming Onlinebackup (Maybe OFFTOPIC)

Jeff Davis <pgsql@j-davis.com> writes:
> On Thu, 2007-09-06 at 12:08 +0100, Heikki Linnakangas wrote:
>> With file-based log shipping, you can get down to 1 second, by using the
>> archive_timeout setting. It will produce a lot of log files with very
>> little content in them, but they will compress well.

> I tried doing a couple pg_switch_xlog() in quick succession, and the WAL
> files that represent less than a second of actual data don't seem much
> more compressible than the rest.

That's because we do not try to zero out a WAL file when recycling it,
so the part after what's been used is not any more compressible than the
valid part.

pg_switch_xlog is not, and was never intended to be, a solution for a
low-latency log-shipping scheme.  The right solution for that is to make
a smarter log-shipping daemon that transmits only part of a WAL segment
at a time.  (As Hannu is getting tired of repeating, you can find a
working example in SkyTools.)
        regards, tom lane