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Hierarchical storage management

Data storage technique

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HSM may also be used where more robust storage is available for long-term archiving, but this is slow to access. This may be as simple as an off-site backup for recovery from disaster.

HSM is a long-established concept, dating back to the beginnings of commercial data processing. The techniques used though have changed significantly as new technology becomes available, for both storage and for long-distance communication of large data sets. The scale of measures such as 'size' and 'access time' have changed dramatically. Despite this, many of the underlying concepts keep returning to favour years later, although at much larger or faster scales.

Implementation

In a typical HSM scenario, data which is frequently used are stored on warm storage device, such as solid-state disk (SSD). Data that is infrequently accessed is, after some time migrated to a slower, high capacity cold storage tier. If a user does access data which is on the cold storage tier, it is automatically moved back to warm storage. The advantage is that the total amount of stored data can be much larger than the capacity of the warm storage device, but since only rarely used files are on cold storage, most users will usually not notice any slowdown.

Conceptually, HSM is analogous to the cache found in most computer CPUs, where small amounts of expensive SRAM memory running at very high speeds is used to store frequently used data, but the least recently used data is evicted to the slower but much larger main DRAM memory when new data has to be loaded.

In practice, HSM is typically performed by dedicated software, such as IBM Tivoli Storage Manager, or Oracle's SAM-QFS.

The deletion of files from a higher level of the hierarchy (e.g. magnetic disk) after they have been moved to a lower level (e.g. optical media) is sometimes called file grooming.

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