On 2/14/2011 2:39 PM, Melvin Davidson wrote:
> Aleksey,
>
> To answer your questions:
>
> 1. So does the database get bigger as it is transfered by Slony?
>
> It doesn't get bigger, but PostgreSQL will write WAL files for all the
> inserts.
> I believe it is the WAL files (16MB each) that is starting to eat up you
> disk space.

I find this unlikely. Slony does nothing but COPY the table content from 
the data provider to the new subscriber. That should not increase the 
size of the pg_xlog directory beyond what is allowed by 
checkpoint_segments and checkpoing_timeout anyway. A COPY that fills 
those too fast should only increase the checkpoint frequency, eventually 
to the point where Postgres is reporting too frequent checkpoints.

>
> 2. how large should the slave's disk be compared to the master's disk?
>
> At least as big as the master. If you intend to use the slave as a
> failover, it is essential
> that the hardware and O/S be as identical as possible.

Only as powerful. Slony was specifically designed to allow cross 
platform and even cross Postgres version replication.


>
> 3. However my database size on the slave is 100 GB, and on the master is
> 58 GB.
>
> How are you determining this? Are you using df -h?
> Or are you doing:
>
> SELECT datname,
> rolname as owner,
> pg_size_pretty(pg_database_size(datname))as size_pretty,
> pg_database_size(datname) as size,
> (SELECT pg_size_pretty (SUM( pg_database_size(datname))::bigint)
> FROM pg_database) AS total,
> ((pg_database_size(datname) / (SELECT SUM( pg_database_size(datname))
> FROM pg_database) ) * 100)::numeric(6,3) AS pct
> FROM pg_database d
> JOIN pg_authid a ON a.oid = datdba
> ORDER BY datname;
>
> Also, make sure autovacuum is enabled on the slave?

It would be interesting to see the part of the subscribers log where it 
starts preparing to copy. Especially if it succeeds in truncating the 
target table or not. A once failed attempt to replicate (for whatever 
reason) could well trigger this odd behavior.


Jan

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