How can I solve postgresql problem after deleting wal files? I turned the archive_mode on on my postgresql configuration for testing a backup server . And since the wal files took lots of disk space, after my test I turned it off and also deleted the wal files. When I tried to restart postgresql I got the following error.

 root@hooshang:/etc/postgresql/9.1/main# /etc/init.d/postgresql restart
 * Restarting PostgreSQL 9.1 database server
 * The PostgreSQL server failed to start. Please check the log output:
 2014-10-16 13:15:28 IRST LOG:  database system was shut down at 2014-10-15 15:51:53 IRST
 2014-10-16 13:15:28 IRST LOG:  could not open file "pg_xlog/00000001000007DC00000037" (log file 2012, segment 55): No such file or directory
 2014-10-16 13:15:28 IRST LOG:  invalid primary checkpoint record
 2014-10-16 13:15:28 IRST LOG:  could not open file "pg_xlog/00000001000007DC00000029" (log file 2012, segment 41): No such file or directory
 2014-10-16 13:15:28 IRST LOG:  invalid secondary checkpoint record
 2014-10-16 13:15:28 IRST PANIC:  could not locate a valid checkpoint record
 2014-10-16 13:15:28 IRST LOG:  startup process (PID 17467) was terminated by signal 6: Aborted
 2014-10-16 13:15:28 IRST LOG:  aborting startup due to startup process failure


How can I fix this issue?
You have corrupted your database by manually deleting files from within the data directory. Never delete files from within the data directory manually. 

Safely removing WAL

If you want to remove WAL, either let the server do it at CHECKPOINT time, or use pg_archivecleanup. Note that the server will remove WAL is no longer needs automatically, unless:


archive_mode is on, but archive_command is failing, so the server keeps on retrying the archive attempts until they succeed or the admin intervenes;
It's still preserved by wal_keep_segments
(in 9.4) It's still needed by a replication slot.


If none of those apply, a CHECKPOINT (either automatic, or manually issued via SQL) will remove all WAL that's not currently required. So you don't have to delete it by hand. 

In unusual circumstances you might need to use pg_archivecleanup, like if you've run out of disk space due to WAL accumulation after archiving has failed for a long time. You might decide to accept that you'll have to re-create your replicas as a result of throwing away WAL they still need and use pg_archivecleanup to free space to get the master running.

But you should never delete WAL segments by hand.

Recovering with archived WAL

If you kept the archived WAL somewhere else, you might just be able to copy the files back into pg_xlog, or create a recovery.conf with a restore_command to do so. See the manual on PITR and log shipping for details.

Recovering without archived WAL

If you don't have a copy of those WAL files somewhere else, you should restore from a backup if you have a recent backup, because you have corrupted your database.

If you do not have a backup, follow the instructions in the corruption wiki page and only once you have made a complete copy of the current state of the database, as a last resort only, use pg_resetxlog to throw away the transaction logs and force the DB to start with incomplete transactions.

You must then pg_dump the database, stop it, initdb a new one, and restore to it. Do not keep using the database you damaged. Never keep using a database you used pg_resetxlog on, and never use it except as a last resort.