delta copies

[Matt McCutchen]

On 9/23/07, WebTent <lists at webtent.net> wrote:
> Well, I am getting matched data, but it just doesn't seem to be matching
> very much considering the small change in file size. I tested one dump
> after another rsyncing in between dumps and got very little matched
> data :(

> mx1# pg_dump -Fc -Upgsql maia > maia.sql

> Total transferred file size: 999709040 bytes
> Literal data: 987800910 bytes
> Matched data: 11908130 bytes

> Since this database is very active as a mail cache, I guess it is
> changing more data than it seems.

Not necessarily.  Depending on how pg_dump works, it could be that
small changes to the database are resulting in unnecessarily large
changes to the dump.  Make sure you are using the uncompressed format
because most compression algorithms defeat the delta-transfer
algorithm almost completely.  Then you might take a look at two
consecutive dumps and check whether records common to both appear in
the same order in each dump.  (If pg_dump is dumping the records in a
different order each time, that would also defeat the delta-transfer
algorithm because no block of several consecutive records could be
matched.)

Matt