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Version Control with Subversion
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Version Control with Subversion - Repository Maintenance - Managing Disk Space

Managing Disk Space

While the cost of storage has dropped incredibly in the past few years, disk usage is still a valid concern for administrators seeking to version large amounts of data. Every additional byte consumed by the live repository is a byte that needs to be backed up offsite, perhaps multiple times as part of rotating backup schedules. If using a Berkeley DB repository, the primary storage mechanism is a complex database system, it is useful to know what pieces of data need to remain on the live site, which need to be backed up, and which can be safely removed. This section is specific to Berkeley DB; FSFS repositories have no extra data to be cleaned up or reclaimed.

Until recently, the largest offender of disk space usage with respect to Subversion repositories was the log files to which Berkeley DB performs its pre-writes before modifying the actual database files. These files capture all the actions taken along the route of changing the database from one state to another—while the database files reflect at any given time some state, the log files contain all the many changes along the way between states. As such, they can start to accumulate quite rapidly.

Fortunately, beginning with the 4.2 release of Berkeley DB, the database environment has the ability to remove its own unused log files without any external procedures. Any repositories created using an svnadmin which is compiled against Berkeley DB version 4.2 or greater will be configured for this automatic log file removal. If you don't want this feature enabled, simply pass the --bdb-log-keep option to the svnadmin create command. If you forget to do this, or change your mind at a later time, simple edit the DB_CONFIG file found in your repository's db directory, comment out the line which contains the set_flags DB_LOG_AUTOREMOVE directive, and then run svnadmin recover on your repository to force the configuration changes to take effect. See the section called “Berkeley DB Configuration” for more information about database configuration.

Without some sort of automatic log file removal in place, log files will accumulate as you use your repository. This is actually somewhat of a feature of the database system—you should be able to recreate your entire database using nothing but the log files, so these files can be useful for catastrophic database recovery. But typically, you'll want to archive the log files that are no longer in use by Berkeley DB, and then remove them from disk to conserve space. Use the svnadmin list-unused-dblogs command to list the unused log files:

$ svnadmin list-unused-dblogs /path/to/repos
/path/to/repos/log.0000000031
/path/to/repos/log.0000000032
/path/to/repos/log.0000000033

$ svnadmin list-unused-dblogs /path/to/repos | xargs rm
## disk space reclaimed!

To keep the size of the repository as small as possible, Subversion uses deltification (or, “deltified storage”) within the repository itself. Deltification involves encoding the representation of a chunk of data as a collection of differences against some other chunk of data. If the two pieces of data are very similar, this deltification results in storage savings for the deltified chunk—rather than taking up space equal to the size of the original data, it only takes up enough space to say, “I look just like this other piece of data over here, except for the following couple of changes”. Specifically, each time a new version of a file is committed to the repository, Subversion encodes the previous version (actually, several previous versions) as a delta against the new version. The result is that most of the repository data that tends to be sizable—namely, the contents of versioned files—is stored at a much smaller size than the original “fulltext” representation of that data.

Note

Because all of the Subversion repository data that is subject to deltification is stored in a single Berkeley DB database file, reducing the size of the stored values will not necessarily reduce the size of the database file itself. Berkeley DB will, however, keep internal records of unused areas of the database file, and use those areas first before growing the size of the database file. So while deltification doesn't produce immediate space savings, it can drastically slow future growth of the database.


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Version Control with Subversion
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