Re: Running vacuumdb -a taking too long

In response to Keaton Adams <kadams(at)mxlogic(dot)com>:

> We are to a point in size and utilization of a set of our Postgres 8.1.17 databases that a vacuumdb -a -z -U postgres is still running after 50 hours and we have to kill it off because the additional i/o is causing queries to stack up. We have archived off as much data as possible (100+ GB) and we are looking at moving to a better hardware configuration to support the increased system activity, but for now I need to know if there is a way to avoid "transaction ID wraparound" if we don't issue a vacuumdb -a to update the datfrozenxid value.
>
> This is a logging type database where data is initially loaded and then "rolled up" into weekly/monthly tables. Many of the tables in the DB don't last for more than seven days, and after a week the majority of the historical tables are static and stay around until they are finally dropped off (based on a retention period), so re-running a vacuum on these older static tables doesn't really gain much since the data never changes.
>
> I have read (and re-read) the documentation and am a bit confused on exactly what needs to happen here:
>
> Since periodic VACUUM runs are needed anyway for the reasons described earlier, it's unlikely that any table would not be vacuumed for as long as a billion transactions. But to help administrators ensure this constraint is met, VACUUM stores transaction ID statistics in the system table pg_database. In particular, the datfrozenxid column of a database's pg_database row is updated at the completion of any database-wide VACUUM operation (i.e., VACUUM that does not name a specific table). The value stored in this field is the freeze cutoff XID that was used by that VACUUM command. All normal XIDs older than this cutoff XID are guaranteed to have been replaced by FrozenXID within that database. A convenient way to examine this information is to execute the query
>
> SELECT datname, age(datfrozenxid) FROM pg_database;
>
> The age column measures the number of transactions from the cutoff XID to the current transaction's XID.
>
> So if, after a table is no longer added to (becomes static), I run a VACUUM against it, the table wouldn't need to be vacuumed again since the tuples and their related transaction ID's never change? Is there a way to set up a vacuum scheme on a table-by-table basis to accomplish the same goal as an all-in-one vacuumdb -a run that I can spread out over time instead of relying on the completion of a single vacuumdb -a command?

I'm not quite sure what the best answer is to your problem, but since
nobody else has suggested anything, here are my thoughts.

First off, can you allocate more maintenance_work_mem? Perhaps that can
speed up vacuum enough.

Secondly, if you VACUUM FREEZE those static tables, it will guarantee
that you'll never lose data from them, even if you hit XID wraparound.
If you VACUUM FREEZE tables one at a time, perhaps you can avoid the
huge performance hit.

Third, while your argument about tables not needing vacuumed again seems
logical, it's simply not how PG functions. Since the XID is database-wide,
it may affect any table. Of course, the FREEZE process will protect tables
from this. This seems to be improved in newer versions of Postgres, so
an upgrade should improve the issue.

Finally, are you really in danger of hitting the wraparound? If you run
the query "SELECT datname, age(datfrozenxid) FROM pg_database;" (as suggested
in the docs) once a day for a few days, does it seems like you're using
up XIDs fast enough to be a danger? If you've got new hardware coming
soon anyway, perhaps you have enough time to now worry about it on the
current hardware?

Hope this helps.

--
Bill Moran
http://www.potentialtech.com
http://people.collaborativefusion.com/~wmoran/