postgres pg_xlog explodes during set tablespace

Question

I'm running postgreSQL 9.6.0 on CentOS 7.2. Occasionally I need to shift data on to a larger tablespace to prevent /var from filling up. When this is running the pg_xlog directory grows rapidly, and recovers very slowly. Periodic CHECKPOINT's subsequently help to kick the xlog clean up into action. However, this laborious severely limits the amount of data I can shift in a short space of time. Upgrades and restarts are very tricky since the database records continuous long running collections from various data sources.

My gut feeling is that SET TABLESPACE prevents updates from WAL being committed, but this doesn't make much sense...

Any help or insight greatly appreciated!

Answer 1

When you move a table to a new table space and your wal_level is above "minimal", then the entire contents of the table are copied to the WAL stream. That is why pg_xlog grows quickly. Shrinking slowly could simply be because your archive_command can't keep up, and WAL files can't be removed until they are archived if archiving is on. What are your settings for wal_level, archive_mode, and archive_command?