SAN Cache: SSD In The SAN - Technology Information

Computer Technology Review,  April, 2000  by Michael Casey

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Solid-State Disk (SSD) modules will be installed as shared file-caching facilities on Storage Area Networks (SANs). These SAN cache appliances will enable rapid growth of the SAN infrastructure by multiplying application performance and by supporting virtual storage addressing. Conversely, SAN architectures will expand the range of applications and environments that can exploit the full benefits of SSD by making it easy to share and easy to manage.

Storage Area Network (SAN) discussions generally extol the benefits of sharing storage devices and subsystems--including disk drives, RAID subsystems, tape drives, automated tape libraries, and perhaps, other storage media such as CD-ROM or DVD. I would like to add another hardware category to the standard SAN architectural sketch: the Solid-State Disk (SSD) file cache appliance or "SAN cache" (Fig 1).

Earlier articles have documented the impact of disk drive access density on I/O performance ("Access Density--Key to Disk Performance" by Randy Kerns, Storage, Inc., Q2 1999) and the use of SSD to multiply application-server performance and scalability ("Disk I/O Performance Scaling: The File Caching Solution" by Michael Casey, www.soliddata.com/whitepapers/ file_caching.html). The latter article introduced a key distinction-- between block caching and file caching (see Sidebar) and described the benefits of server-attached SSD as a performance multiplier in transaction-intensive applications such as e-mail, messaging, and e-business.

This article outlines the synergies between SSD and SAN technologies: SSD will become a key-enabling technology for SAN performance and scalability and SANs will enable enterprises add system integrators to exploit the full potential of SSD as a performance multiplier. These benefits will be most fully realized when the SAN infrastructure is automatically configured and managed as part of a virtual storage architecture.

SSD As Enabling Technology For SANS

When an intelligent solid-state disk subsystem is added to a SAN, it becomes a shared file caching facility for the application servers attached to the storage network. It also becomes available as part of the storage infrastructure and system integrators and ISVs can use the file cache to enable functionality and performance that would be impossible to achieve with mechanical disk drives alone.

One key benefit is modular scalability of capacity and performance in the storage infrastructure. In a modular SAN architecture, a storage administrator or system integrator can configure the desired amount of capacity by adding disk drives or disk array modules. By adding SSD modules that are separate from the disk drive arrays, a SAN architect can independently "dial in" the desired amount of performance for transaction-intensive applications. Fig 2 illustrates this concept of managing capacity and performance as two separate dimensions with a separate "control dial" for each. Initially, this is a metaphor for a manual configuration process; however, the process will ultimately become an automated capability of virtual storage architectures.

By exploiting an optimized mix of RAID and SSD modules, a SAN architecture can deliver cost-effective configurations for a wide range of applications--including those for which a single, monolithic approach is ineffective or needlessly expensive. Fig 3 positions a number of applications in terms of their high-water-mark requirements for response time and for bandwidth.

SANs Enhance The Scope Of SSD

SAN developments will make SSD file caching easier to connect and share, thus making SSD cost-effective for a wider range of operating environments and applications. Fibre Channel provides a number of benefits, even when it is used simply as a replacement for SCSI connections in server-attached storage configurations (which is what most server vendors support today). These FC benefits include longer supported distances from server to storage and improved robustness and flexibility in hot-plug configurations.

The benefits of Fibre Channel are more fully exploited in server clusters that employ FC connections between the server nodes and a shared storage facility. A cluster architecture can use a shared SSD to make key files available to all servers in the cluster; this provides shared, non-volatile storage while avoiding the mechanical access latencies that that are introduced by shared disk storage. High-availability cluster software--such as VERITAS Cluster Server and Hewlett-Packard's MC/ServiceGuard--will dramatically increase application recovery speed by maintaining file system journals (write logs) and related data structures on a shared, high-speed file cache.

As the industry moves toward switched-fabric SANs with heterogeneous server and operating system support, the deployment of SAN management software will enable enterprises to realize the benefits of file caching in a much broader range of applications. Whereas server-attached SSD is best suited to applications that can justify investment in a dedicated file-caching subsystem, a shared SAN cache can be used by applications that could not justify dedicated file caches. These include smaller NT and Linux servers, each of which might need only a fraction of the capacity provided by a robust SSD product. In a SAN, many servers can share one file cache and storage management software can allocate appropriate amounts to each server as the workload changes.