Tablespaces are designed to allow Postgres clusters to be spread across multiple storage devices. Create tablespace creates a symbolic link in the pg_tblspc directory in the cluster's data directory pointing to the newly-created tablespace directory.

Unfortunately, though there is a command to move tables and indexes between tablespaces, there is no command to move tablespaces to different directories. However, since Postgres 9.2, the process of moving tablespaces is quite simple:

1. Record the oid of the tablespace you want to move
2. Shut down the Postgres cluster
3. Move the tablespace directory, either within the same file system or to a different file system
4. Update the oid symbolic link that represents the moved tablespace to the new tablespace directory location
5. Restart the server

See an example of moving a tablespace on Postgres Rocks. >>

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