Re: Performance opportunities for write-only audit tables? - Mailing list pgsql-general

Hi,

The two main techniques we use are.

1. Create a script to pass the -t param to pg_dump to exclude the log tables.  The idea here is to backup the rest of your DB to one backup regime and the log tables to another. We set it up so at the end of the day the current log table is backed up and loaded into an backup archive, then we vacuum freeze the log table.  The benefits are that once each log table is "Closed" (meaning you will no longer be writing to that log table ever again), is backed up once only.... ever...   It speeds housekeeping up, and your daily backups are much smaller.

2. Use  UNLOGGED in the log table creates.

Have fun.

Andrew

On Fri, 25 May 2018 at 02:55 Paul Jungwirth <pj@illuminatedcomputing.com> wrote:

Hi,

A project of mine uses a trigger-based approach to record changes to an
audit table. The audit table is partitioned by month (pg 9.5, so
old-fashioned partitioning). These tables are write-heavy but
append-only and practically write-only: we never UPDATE or DELETE, and
we seem to consult them only a few times a year. But they are enormous:
bigger than the rest of the database in fact. They slow down our
backups, they increase WAL size and streaming replication, they add to
recovery time, they make upgrades more time-consuming, and I suppose
they compete for RAM.

This is all on an AWS EC2 instance with EBS storage. We also run a warm
standby with streaming replication.

Since these tables are so different from everything else, I'm wondering
what opportunities we have to reduce their performance cost. I'm
interested both in practical high-bang-for-buck changes, but also in
harder just-interesting-to-think-about last-resort approaches. Here are
a few ideas of my own, but I'm curious what others think:

We already have no indexes or foreign keys on these tables, so at least
there's no cost there.

Since they are already partitioned, we could move old data to offline
storage and drop those tables. This feels like the biggest, easiest win,
and something we should have done a long time ago. Probably it's all we
need.

Put them on a different tablespace. This one is also pretty obvious, but
aside from using a separate disk, I'm curious what other crazy things we
could do. Is there any per-tablespace tuning possible? (I think the
answer within Postgres is no, but I wish we could change the settings
for wal_level, or exclude them from replication, or something, so I'm
wondering if we could achieve the same effect by exploiting being on a
separate filesystem.) Maybe put the tablespace on some FUSE filesystem
to get async writes? Or just pick different mount options, e.g. on ext4
lazytime,dealloc,data=writeback? I don't know. Or at a different level:
change the triggers so they call a custom function that uses a new
thread to store the audit records elsewhere. Maybe these ideas are all
too risky, but I think the organization is fine with slightly relaxed
durability guarantees for this data, and anyway I'm just curious to have
a list of possibilities before I categorize anything as too crazy or
not. :-)

If we upgraded to pg 10 we could use logical replication and leave out
the audit tables. That is appealing. Even without upgrading, I guess we
could replace those tables with postgres_fdw ones, so that they are not
replicated? Has anyone else used that trick?

Thanks!

--
Paul              ~{:-)
pj@illuminatedcomputing.com