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Version Control with Subversion
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Version Control with Subversion - Repository Maintenance - Repository Recovery

Repository Recovery

As mentioned in the section called “Berkeley DB”, a Berkeley DB repository can sometimes be left in frozen state if not closed properly. When this happens, an administrator needs to rewind the database back into a consistent state.

In order to protect the data in your repository, Berkeley DB uses a locking mechanism. This mechanism ensures that portions of the database are not simultaneously modified by multiple database accessors, and that each process sees the data in the correct state when that data is being read from the database. When a process needs to change something in the database, it first checks for the existence of a lock on the target data. If the data is not locked, the process locks the data, makes the change it wants to make, and then unlocks the data. Other processes are forced to wait until that lock is removed before they are permitted to continue accessing that section of the database.

In the course of using your Subversion repository, fatal errors (such as running out of disk space or available memory) or interruptions can prevent a process from having the chance to remove the locks it has placed in the database. The result is that the back-end database system gets “wedged”. When this happens, any attempts to access the repository hang indefinitely (since each new accessor is waiting for a lock to go away—which isn't going to happen).

First, if this happens to your repository, don't panic. The Berkeley DB filesystem takes advantage of database transactions and checkpoints and pre-write journaling to ensure that only the most catastrophic of events [17] can permanently destroy a database environment. A sufficiently paranoid repository administrator will be making off-site backups of the repository data in some fashion, but don't call your system administrator to restore a backup tape just yet.

Secondly, use the following recipe to attempt to “unwedge” your repository:

  1. Make sure that there are no processes accessing (or attempting to access) the repository. For networked repositories, this means shutting down the Apache HTTP Server, too.

  2. Become the user who owns and manages the repository. This is important, as recovering a repository while running as the wrong user can tweak the permissions of the repository's files in such a way that your repository will still be inaccessible even after it is “unwedged”.

  3. Run the command svnadmin recover /path/to/repos . You should see output like this:

    Repository lock acquired.
    Please wait; recovering the repository may take some time...
    
    Recovery completed.
    The latest repos revision is 19.
    

    This command may take many minutes to complete.

  4. Restart the Subversion server.

This procedure fixes almost every case of repository lock-up. Make sure that you run this command as the user that owns and manages the database, not just as root. Part of the recovery process might involve recreating from scratch various database files (shared memory regions, for example). Recovering as root will create those files such that they are owned by root, which means that even after you restore connectivity to your repository, regular users will be unable to access it.

If the previous procedure, for some reason, does not successfully unwedge your repository, you should do two things. First, move your broken repository out of the way and restore your latest backup of it. Then, send an email to the Subversion user list (at ) describing your problem in detail. Data integrity is an extremely high priority to the Subversion developers.


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Version Control with Subversion
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