SAN Cache: SSD In The SAN - Technology Information

Computer Technology Review, April, 2000 by Michael Casey

The expanded range of applications will also include applications that only need a fast file cache for an occasional workload spike such as a month-end financial close that runs for 30 hours and needs to complete in less than three hours. It might be difficult to justify a dedicated file cache to speed up the month-end close, but it will be easy to justify allocating part of a shared SSD facility to that application for a few hours each month. The shared SAN connection, together with SAN management software will make this dynamic allocation easy enough for widespread adoption. Thus, file caching in the form of a shared SAN cache will be deployed to serve a much wider range of servers and applications. These developments will be further enhanced by virtual storage architectures.

Virtual Storage Architectures

The holy grail of SAN evolution is the Virtual Storage Architecture (VSA) in which SAN management software presents virtual disk volumes to the application servers and maps those virtual address ranges to physical storage devices connected through the SAN. Early examples include storage subsystems and SAN domain servers developed by XIOtech (recently acquired by Seagate) and ConvergeNet (recently acquired by Dell Computer). Other server and storage vendors such as Compaq Computer and Sun Microsystems are also developing virtual storage architectures.

Ultimately, virtual storage architectures will accept high-level requests from a SAN administrator and will automatically allocate physical resources based on storage policies defined for each class of storage. For example, an administrator might use a storage control console to request creation of a 50GB virtual disk volume that can deliver 10,000 I/Os per second (at an 8KB block size) and a maximum response time of one millisecond. The management software would, then, configure the appropriate combination of physical resources available on the SAN such as fast disk, tape storage, and SSD.

Virtual storage architectures will make SSD easy to configure and easy to use. For example, VSA software facilities and services will enable easy migration of data from slow storage to fast storage without disrupting application availability.

SSD will also be a crucial component of the VSA infrastructure. Effective operation of a VSA requires very fast conversion of logical addresses to physical addresses. In a distributed SAN, the obvious place to store the address lookup tables will be a shared, high-speed SAN cache.

In the future, as Storage Area Networks (SANs) are widely deployed and supported by sophisticated storage management tools--such as virtual storage architectures and policy--based storage management consoles--solid-state file cache will become an easily managed, shared facility on the SAN. As such, it will become attractive and cost-effective for architectural deployment in a wide range of applications.

Michael Casey is the vice president of marketing at Solid Data Systems (Santa Clara, CA).

File Caching With SSD