Postgres Gotchas


Filed under: database, upgrade, sharedmemory, featured

I run postgres locally for development on OS X (mountain lion). I’ve run into a few issues and am documenting solutions here.

Issue 1: Insufficient Shared Memory

Postgres requires sufficient shared memory or it won’t start. If there isn’t enough, postgres server will fail to start and in server.log you will see this:

FATAL:  could not create shared memory segment: Cannot allocate memory
DETAIL:  Failed system call was shmget(key=5432001, size=3391488, 03600).
HINT:  This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory or swap space, or exceeded your kernel's SHMALL parameter.  You can either reduce the request size or reconfigure the kernel with larger SHMALL.  To reduce the request size (currently 3391488 bytes), reduce PostgreSQL's shared memory usage, perhaps by reducing shared_buffers or max_connections.

There are two ways to correct this problem.

  1. Up the shared memory configuration.

    On mountain lion, you could create or modify your /etc/sysctl.conf so it has this:

     zlu@zlu-mba:~/projects/me/blog-zlu (master)$ cat /etc/sysctl.conf

    Make sure shmmax is large enough for postgres and other software that requires shared memeory. To figure out how much shared memeory you will need for postgres, take a look at postgresql.conf (probably under /usr/local/var/postgres)

     max_connections = 20                    # (change requires restart)
     # Note:  Increasing max_connections costs ~400 bytes of shared memory per
     # connection slot, plus lock space (see max_locks_per_transaction).

    More details can be found here: Upgrade Postgres

  2. Reduce resource requirements In postgresql.conf you could search for “shared memory” and see all resources (such as max_connections and max_prepared_transactions). Reduce the requirements as you see fit.

Issue 2: Upgrading from 9.1.x to 9.2.x

If you need to migration database, you may run into path issues.

Here is the steps that work for me:

  1. Backup BEFORE you install 9.2 (with homebrew). This is important if you want to avoid the hussle later with two versions of pg_ctl stomping onto each other. While the 9.1.x server is running, use pg_dumpall to export the database.
  2. Stop 9.1.x server
  3. mv the data directory (e.g. /usr/local/var/postgres) to postgres.bk.
  4. Install postgres 9.2.x (with homebrew).
  5. Create symlink for plist file for launchctl (it is named differently than 9.1 so you will need to do it again). The post-install instruction of homebrew contains the exact command. cp /usr/local/Cellar/postgresql/9.2.1/homebrew.mxcl.postgresql.plist ~/Library/LaunchAgents/ And remove the plist file that belonged to 9.1.x
  6. initdb (this creates the 9.2.x version of the database).
  7. Import the postgres data backup from the previous pg_dumpall command.
  8. Load plist: launchctl load -w ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist

Migrating Postgres

Issue 3: Rails Server Fails to Start

If you are doing Rails development, after upgrade postgres and issue ‘rails s’ for your project, you may see this error which fails Rails server from starting:

PGError (could not connect to server: Permission denied
    Is the server running locally and accepting
    connections on Unix domain socket "/var/pgsql_socket/.s.PGSQL.5432"?

Since postgres 9.2, the unix socket file has changed to /var/pgsql_socket/alt. The pg gem is compiled against the older version of postgres (9.1.x) so it looks for the socket file in wrong location. The easist thing to do is to uninstall and reinstall pg gem.