On 22/09/14 02:24, Michael Paquier wrote:
On Thu, Sep 4, 2014 at 11:33 PM, Michael Paquier
Taking a dump consistent with a replication slot is useful for online
upgrade cases first, because you can simply run pg_dump, have a slot
created, and get as well a state of the database consistent with the
slot creation before replaying changes in a way or another. Using
that, a decoder that generates raw queries, and a receiver able to
apply changes on a remote Postgres server, it is possible to get a
kind of live migration solution from a Postgres instance to another
for a single database, as long as the origin server uses 9.4. Making
the receiver report write and flush positions makes also possible the
origin server to use synchronous replication protocol to be sure that
changes got applied on remote before performing a switch from the
origin to the remote (that may actually explain why multi-syncrep
would be useful here for multiple databases). Also, I imagine that
users could even use this tool in pg_dump for example to do some post
processing on the data dumped in accordance to the decoder plugin
before applying changes to a remote source.
Now, this is done with the addition of two options in pg_dump to
control the logical slot creation:
- --slot to define the name of the slot being created
- --plugin-name, to define the name of the decoder plugin
And then you can of course do things like that:
# Raw data dump on a slot
$ pg_dump --slot bar --plugin-name test_decoding
# Existing parallel dump not changed:
$ pg_dump -j 4 -f data -F d
# Parallel dump on a slot
$ pg_dump -j 4 --slot bar --plugin-name test_decoding -f data -F d
Wouldn't it be better to have the slot handling done outside of pg_dump
by whatever replication solution you use and just have pg_dump accept
the snapshot as input parameter? I am not sure how much I like pg_dump
creating the slot. I am aware that you need to have the replication
connection open but that's IMHO just matter of scripting it together.
--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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