The Mass Storage system (also known as SAM-FS or /ms) is intended to be used for archiving files and storing very large files, files that are too large to fit within your AFS quota.

SAM-FS is similar to an ordinary disk file system in that it keeps an inode (for recording data location, etc.) and data blocks for each file. For the user of mass storage, this file system appears to be a subdirectory of the user's AFS home directory. Files can be moved in and out of mass storage by using simple Unix/Linux commands such as cp and mv or by using sftp and scp .

SAM-FS is different from ordinary disk systems in that it keeps data blocks on tapes, while the inodes remain on disk. When a file is created, an inode is immediately created and the data goes to the SAM-FS disk cache. If the file stays unmodified for a few hours, it will be archive copied to tape. The files are written to tape as tar archives; SAM-FS takes a number of files and directories and uses the standard Unix/Linux tar command to create a single tar file (archive). The tape drive hardware compresses the data as it is written to tape. SAM-FS copies the data to two different tapes to ensure that we have a backup copy of every file. If one tape goes bad, we can still retrieve your data.

When the SAM-FS disk cache is 90% full, SAM-FS automatically does a release: it flushes the data blocks of files that have already been written to tape until the disk cache is only 70% full. When a file that has been flushed is accessed it will take at least one minute for the data to be brought back into the SAM-FS disk cache.

Every 24 hours, the inodes are backed up to a location other than SAM-FS space. Therefore, if we experience a disk crash related to SAM-FS, we can restore the inode table from backup. This will restore every file that was written to tape (archived) and had its inode backed up (this means the file must have been unmodified for a minimum of 24 hours and a maximum of 48 hours).