One way IT administrators have compensated is to tier application data storage and use faster, more expensive HDDs for some items and slower, less expensive HDDs for others. This is known as hierarchical storage management (HSM). The goal of HSM is to increase service levels to critical applications and data sets, while reducing the overall cost of data storage.
In general, the lower the number of the tier in a tiered storage hierarchy, the more expensive the storage media and the less time it takes to retrieve data on that tier. An enterprise that requires selected applications to be accessed very quickly may choose to use expensive solid-state storage in its very fastest tier, which some data storage professionals call Tier 0.
The addition of Tier 0 to the data storage hierarchy represents a change from moving less active data to slower, less expensive storage to a focus on moving more active data to faster, more expensive storage.
Here is an example of what a storage hierarchy that incorporates Tier 0 might look like:
History of storage tiers
Before the common use of solid-state drives (SSDs), Tier 0 used a RAM disk or assigned a block of server RAM to function as a virtual disk drive. But using system RAM for this task takes away from the RAM available for computation. While the first SSDs were much more expensive than today's products, the cost per gigabyte was still lower than using system RAM, and they allowed for greater amounts of storage for higher-speed access than the HDDs used in Tier 1. RAM disks also require constant power, while SSDs are made of nonvolatile flash memory.
One of the early products on the market was the ioDrive, a separate card that plugged into a server from Fusion-io and provided flash memory that appeared as an SSD to the storage system. Automated storage tiering (AST) software is now commonly included in hybrid storage arrays that have SSDs and HDDs. AST software ensures the most frequently accessed data, known as hot data, is moved to the speedy Tier 0. In large enterprise storage systems, entire all-flash arrays can be assigned as Tier 0 storage, with hybrid arrays as Tier 1, HDD arrays as Tier 2 and tape or slower, inexpensive HDD arrays as Tier 3.
Performance is king, cost is queen
While any storage system can benefit from the speed boost of Tier 0, it finds the greatest need in anything that relies on high-performance computing. Typical HPC applications that use high-transaction databases include medical research, security analysis, financial services and big data analytics.
If performance was the only factor to consider, organizations would have nothing but all-flash arrays in their data storage structures. But cost is also an issue, and that is where HSM becomes a vital player. By establishing a tiered hierarchy of what storage is needed and how often, the least-expensive storage product for each tier can be implemented. Adding to the reduction in cost is AST software that allows an organization to determine just how much frequently accessed data needs to be called hot and how much Tier 0 storage it needs.