How to Implement your own DirectoryStorage Backup Procedure
Taking a backup of a DirectoryStorage consists of forcing the storage into snapshot mode, then taking a copy of the files in the database directory. The essential directories are HOME/config, and HOME/A.
While in snapshot mode the storage still has write access to the database; writes go into the directories HOME/journal and HOME/B. You do not need to take copes of these in your backup.
To implement your own full backup procedure:
- Take a copy of the HOME/config directory. This directory is small, and is not modified by a running DirectoryStorage.
- If you know the storage is running, force it into snapshot mode. Lock the file HOME/misc/sublock to ensure that noone else will modify the directory during your backup, then check for the existense of a file HOME/misc/snapshot to prove that the database really is a snapshot. For more details on this process, see doc/snapshot.txt.
- Take a full copy of the HOME/A directory. This directory contains the full database content, complete as of the time where it entered snapshot mode.
- DO NOT backup the directories HOME/B or HOME/journal. These directories contain data from transactions that were committed after entering snapshot mode.
- Unlock HOME/misc/sublock and leave snapshot mode.
To implement your own incremental live backup:
- If your incremental backup system uses timestamps, record the time now. You will need it to perform your next incremental backup.
- Force it into snapshot mode, as described above.
- Take an incremental copy of HOME/A using the last-modified time, or other means. An incremental backup need not record which files have been deleted.
- Leave snapshot mode
It is possible for a storage to be shut down and the file misc/snapshot does not exist. In this state the main directory is not a coherent snapshot. This can happen if:
- The storage process terminates without calling .close()
- The storage process calls .close() but the storage can not enter snapshot mode promptly. This may happen if it is closed immediately after leaving snapshot mode.
- The quick_shutdown configuration option has been turned on.
When the DirectoryStorage is shut down in this state there are two options for taking a backup:
- Start the storage. The storage will roll forwards any complete transactions, and you can then proceed as normal. This is the easiest approach.
- Take a copy of everything, including HOME/B and HOME/journal. Note that this option is incompatible with incremental backups. This option is appropriate if something has crashed and your want a copy of the wreckage. You can only do this if DirectoryStorage is shut down, or you are using a system like LVM to give a filesystem-level snapshot.