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. 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. This limitation might be
removed in the future (see issue 155).

Note that:

• 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.

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 two large disadvantages:

• backups and restores always have to be made with a special program that takes care of
the hardlinking. The backup 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 just introduced by the backup program to
safe space)

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

OPTIONS

       The s3qlcp command accepts the following options:

          --debug
                 activate debugging output

          --quiet
                 be really quiet

          --version
                 just print program version and exit

EXIT STATUS

       s3qlcp returns exit code 0 if the operation succeeded and 1 if some error occurred.

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