Josh Berkus: Settings for a fast pg_restore

One thing which we do a lot for clients is moving databases from one server to another via pg_dump and pg_restore.  Since this process often occurs during a downtime, it's critical to do the pg_dump and pg_restore as quickly as possible.  Here's a few tips:

• Use the -j multiprocess option for pg_restore (and, on 9.3, for pg_dump as well).  Ideal concurrency is generally two less than the number of cores you have, up to a limit of 8.  Users with many ( > 1000) tables will benefit from even higher levels of concurrency.
• Doing a compressed pg_dump, copying it (with speed options), and restoring on the remote server is usually faster than piping output unless you have a very fast network.
• If you're using binary replication, it's faster to disable it while restoring a large database, and then reclone the replicas from the new database.  Assuming there aren't other databases on the system in replication, of course.
• You should set some postgresql.conf options for fast restore.

"What postgresql.conf options should I set for fast restore?" you ask?  Well, let me add a few caveats first:

• The below assumes that the restored database will be the only database running on the target system; they are not safe settings for production databases.
• It assumes that if the pg_restore fails you're going to delete the target database and start over.
• These settings will break replication as well as PITR backup.
• These settings will require a restart of PostgreSQL to get to production settings afterwards.

shared_buffers = 1/2 of what you'd usually set
maintenance_work_mem = 1GB-2GB
fsync = off
synchronous_commit = off
wal_level = minimal
full_page_writes = off
wal_buffers = 64MB
checkpoint_segments = 256 or higher
max_wal_senders = 0
wal_keep_segments = 0
archive_mode = off
autovacuum = off
all activity logging settings disabled

Some more notes:

• you want to set maintenance_work_mem as high as possible, up to 2GB, for building new indexes.  However, since we're doing concurrent restore, you don't want to get carried away; your limit should be (RAM/(2*concurrency)), in order to maintain somewhat of an FS buffer.  This is a reason why you might turn concurrency down, if you have only a few large tables in the database.
• checkpoint_segments should be set high, but requires available disk space, at the rate of 1GB per 32 segments.  This is in addition to the space you need for the database.

Have fun!