CSYNC User Guide

It is often the case that we have multiple copies (called replicas) of a filesystem or part of a filesystem (for example on a notebook and on a desktop computer). Changes to each replica are often made independently and as a result they do not contain the same information. In that case a file synchronizer is used to make them consistent again, without losing any information.

The goal is to detect conflicting updates (files which has been modified) and propagate non-conflicting updates to each replica. If there are no conflicts left we are done and the replicas are identical.

This section describes some basics you might need to understand how file synchronization works.

A path normally refers to a point with a set of files which should be synchronized. It is specified relative to the root of the replica. The path is just a sequence of names separated by /.

csync always uses the absolute path. This could be /home/gladiac or for sftp sftp://gladiac:secret@myserver/home/gladiac.

The contents of a path could be a file, a directory or a symbolic link (symbolic links are not supported yet). To be more precise, if the path refers to:

csync keeps a record of each path which has been successfully synchronized. The path gets compared with the record and if it has changed since the last synchronization, we have an update. This is done by comparing the modification or change (modification time of the metadata) time.

A path is conflicting if it fulfills the following conditions:

The main goal of a file synchronizer is correctness. It changes whole or separated pieces of a users file system. So a user is not able to monitor the complete file synchronization process, and the synchronizer is in a position where it can damage the file system. It is important that the implementation behaves correctly under all conditions, even if there is an unexpected error (for example, disk full).

On problem concerning correctness is the handling of conflicts. Each file synchronizer tries to propagate conflicting changes to the other replica. At the end both replicas should be identical. There are different strategies to fulfill these goals.

csync is a 3-phase file synchronizer. The desicion for this design was that user interaction should be possible and it should be easy to understand the process. The 3 phases are update detection, reconciliation and propagation. These will be described in the following sections.

There are differnt strategies to do update detection. csync uses a state-based modtime-inode update detector. This means it uses a the modification time to detect updates. It doesn't require much resources. A record of each file is stored in a database (called statedb) and compared with the current modification time during update detection. If the file has changed since the last synchronization a instruction is set to evaluate it during the reconcilation phase. If we don't have a record for a file we invastigate, it is marked as new.

There is a problem to detect renaming of files. This is sovled by the record we store in the statedb too. If we don't find the file by the name in the database we search for the inode number. If the inode number is found then the file has been renamed.

The most improtant component is the update detector, because the reconciler depends on it. The correctness of reconciler is mandatory because it can damage a filesystem. It decides which file:

A wrong decision of the reconciler leads in most cases to a loss of data. So there are several conditions a the file synchronizer has to follow.

For conflict resolution several different algorithms could be implemented. The most common algorithm are the merge and and the conflict algorithm. The first is a batch algortihm and the second is one which needs user interaction.

The merge algorithm is an algorithm which doesn't need any user interaction. It is simple and used for example by Microsoft for Roaming Proflies. If it detects a conflict (the same file changed on both replicas) then it will use the most recent file and overwrite the other. This means you can loose some data, but normally you want the latest file.

This is not implemented yet.

If a file has a conflict the user has to decicde which file should be used.

The next instance of the file synchronizer the propagator. It uses the calculated records to apply them on the current replica.

The propagator uses a 2-phase-commit mechanism to simulate an atomic filesystem operation.

In the first phase we copy the file to a temporary file on the opposite replica. This has the advantage that we can check if file which has been copied to the opposite replica has been transfered successfully. If the connection gets interruppted during the transfer we still have the orignal states of the file. This means no data will be lost.

In the second phase the the file on the opposite replica will be overwritten by the temporary file.

After a successfull propagation we have to merge the trees to reflect the current state of the filesystem tree. This updated tree will be written as a journal into a database. The database is called the state database. It will be used during the update detection of the next synchronization. See above.

This is a really important topic. The file synchronizer should not crash and if it has crashed, there should be no loss of data. To achieve this goal there are several mechanisms to prevent this. These mechnanisms will be discussed in the following sections.

The synchronization process can be interrupted by different events, this can be:

That no data will be lost due to the occurance we enforce the following invariant:

This means, that the original content can not be incorrect, no data can't be lost until we overwrite it after a successful synchronization. So each interupted synchronization process is a partial sync and can be continued and completed by simply running csync again. The only problem could be an error of the filesystem. So we reach this invariant only approximatly.

With the Two-Phase-Commit we check the file size after the file has transferred. So we can detect transfer errors. Better would be a transfer protocol with checksums. We may do this in the future.

Future filesystems, like btrfs, will help to compare checksums instead of the filesize. This will make the synchronization safer. This doen't mean that it is unsafe now, but checkums are better then just filesize checks.

It could be possible, that the state database get corrupted. If this happens all files get evaluated. In this case the file synchronizer wont delete any file, but it could occur that deleted files will be restored from the other replica.

To prevent a corruption or loss of the database if an error occurs or the user forces an abort, the synchronizer is working on a copy of the database and will use a Two-Phase-Commit to save it at the end.

See the README and INSTALL files for install prerequisites and procedures. Packagers take a look at Appendix B: Packager Notes.

The synopsis of the commandline client is

It synchronizes the content of SOURCE with DESTINATION and vice versa. The DESTINATION can be a local directory or a remote file server.

To synchronize two local directories:

Two synchronizer a local directory with an smb server, use

If you use kerberos you don't have to specify a username or a password. If you don't use kerberos, the commandline client will ask about the user and the password. If you don't want to be ask, you can specify it on the commandline:

If you use the sftp protocol and want to specify a port, you do it the following way:

The remote destination is supported by plugins. By default csync ships with smb and sftp support. For more information, see the manpage of csync(1).

pam_csync is a PAM module to provide roaming home directories for a user session. This module is aimed at environments with central file servers a user wishes to store his home directory. The Authentication Module verifies the identity of a user and triggers a synchronization with the server on the first login and the last logout. More information can be found in the manpage of the module pam_csync(8).

Read the README, INSTALL and FAQ files (in the distribution root directory).