Ubuntu Manpage: s3qlcp - Copy-on-write replication on S3QL file systems

NAME

       s3qlcp - Copy-on-write replication on S3QL file systems

SYNOPSIS

          s3qlcp [options] <source-dir> <dest-dir>

DESCRIPTION

S3QL is a file system for online data storage. Before using S3QL, make sure to consult the
full documentation (rather than just the man pages which only briefly document the
available userspace commands).

The s3qlcp command duplicates the directory tree source-dir into dest-dir without
physically copying the file contents. Both source and destination must lie inside the
same S3QL file system.

The replication will not take any additional space for the file contents. Metadata usage
will increase proportional to the amount of directory entries and inodes. Only if one of
directories is modified later on, the modified data will take additional storage space.

s3qlcp can only be called by the user that mounted the file system and (if the file system
was mounted with --allow-other or --allow-root) the root user.

After the replication, both source and target directory will still be completely ordinary
directories. You can regard <src> as a snapshot of <target> or vice versa. However, the
most common usage of s3qlcp is to regularly duplicate the same source directory, say
documents, to different target directories. For a e.g. monthly replication, the target
directories would typically be named something like documents_January for the replication
in January, documents_February for the replication in February etc. In this case it is
clear that the target directories should be regarded as snapshots of the source directory.

Exactly the same effect could be achieved by an ordinary copy program like cp -a. However,
this procedure would be orders of magnitude slower, because cp would have to read every
file completely (so that S3QL had to fetch all the data over the network from the backend)
before writing them into the destination folder (at which point S3QL would de-duplicate
the data).

Snapshotting vs Hardlinking
Snapshot support in S3QL is inspired by the hardlinking feature that is offered by
programs like rsync or storeBackup. These programs can create a hardlink instead of
copying a file if an identical file already exists in the backup. However, using hardlinks
has disadvantages:

• backups and restores always have to be made with a special program that takes care of
the hardlinking. The backups must not be touched by any other programs (they may make
changes that inadvertently affect other hardlinked files)

• special care needs to be taken to handle files which are already hardlinked (the restore
program needs to know that the hardlink was not introduced by the backup program)

S3QL snapshots do not have these problems, and they can be used with any backup program.

OPTIONS

       The s3qlcp command accepts the following options:

          --log <target>
                 Destination for log messages. Specify none for standard output or syslog for the
                 system  logging  daemon.  Anything  else will be interpreted as a file name. Log
                 files will be rotated when they reach 1 MiB, and at most 5 old log files will be
                 kept. Default: None

          --debug-modules <modules>
                 Activate  debugging  output  from  specified  modules  (use  commas  to separate
                 multiple modules, 'all' for everything). Debug messages will be written  to  the
                 target specified by the --log option.

          --debug
                 Activate  debugging output from all S3QL modules. Debug messages will be written
                 to the target specified by the --log option.

          --quiet
                 be really quiet

          --version
                 just print program version and exit

EXIT CODES

       s3qlcp may terminate with the following exit codes:

       0      Everything went well.

       1      An unexpected error occurred. This may indicate a bug in the program.

       2      Invalid command line argument or configuration file key.

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