SAM and QFS

SAM and QFS

SAM and QFS are different things but are closely coupled. QFS is Sun’s cluster filesystem, meaning that the same filesystem may be simultaneously mounted by multiple systems. SAM is a hierarchical storage manager; it allows a set of disks to be used as a cache for a tape library. SAM and QFS are designed to work together, but each may be used separately.

QFS has some interesting features. A QFS filesystem may span multiple disks with no extra LVM needed to do striping or concatenation. When multiple disks are used, data may be striped or round-robined. Round-robin allocation means that each file is written to one or two disks in the set. This is useful since, unlike striping, participation by all disks is not needed to fetch a file – each disk may seek totally independently. QFS also allows metadata to be separated from data. In this way, a few disks may serve the random metadata workload while the rest serve a sequential data workload. Finally, as mentioned before, QFS is an asymmetric cluster filesystem.

QFS cannot manage its own RAID, besides striping. For this, you need a hardware controller, a traditional volume manager, or a raw ZFS volume.

SAM makes a much larger backing store (typically a tape library) look like a regular UNIX filesystem. This is accomplished by storing metadata and often-referenced data on disk, and migrating infrequently used data in and out of the disk cache as needed. SAM can be configured so that all data is staged out to tape, so that if the disk cache fails, the tapes may be used like a backup. Files staged off of the disk cache are stored in tar-like archives, so that potentially random access of small files can become sequential. This can make further backups much faster.

QFS may be used as a local or cluster filesystem for large-file intensive workloads like Oracle. SAM and QFS are often used for huge data sets such as those encountered in supercomputing. SAM and QFS are optional products and are not cheap, but they have recently been released into OpenSolaris.

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