Maximizing storage ROI with fast external cache.

Your RAID system has a problem. It isn't that the system controller is going to fail. It isn't that failed disk drives can't be replaced. It's that the system's performance doesn't scale. The RAID, perhaps only a few years old, is throttled by demands on its performance. Yes, additional spindles may add a hundred or so I/Os per second each, but can this keep pace with the increasing demand of applications and users on storage? Most likely, the answer is "no." This means the RAID array is next in line for the dreaded forklift upgrade. For you, ROI is spelled "e-b-a-y".

Fortunately, storage manufacturers are increasingly focused on storage performance. From the disk drive to the distributed RAID, cache (as in RAM) is the secret ingredient in next generation storage performance. But the increase in cache is happening in small quantities in all but the most massive monolithic solutions. What the storage industry has been missing is a way to combine high-speed cache with low-cost RAID storage solutions. Even at the enterprise level, monolithic RAID solutions will become the way of the past. Flexible, high performing, distributed and inexpensive storage solutions offer the best storage return on investment.

The monolithic RAID is the product of a simple progression: small to big. You start with small controllers, small cache and small spindle counts and innovate by creating systems with big controllers, big cache and big spindle counts. This approach has two major disadvantages: expense and performance scalability. For over a decade, if you needed enterprise performance you had no other choices. Increasingly, innovative companies are developing smarter, more scalable solutions. These solutions isolate and distribute the components of their architecture in order to flexibly address application performance requirements.

The first step in the move to distributed storage is separating the controllers from the disk storage. Many RAID storage companies have taken this step. Separating the controllers from the disk storage allows a company to choose the number of ports and the processing power dedicated to their storage and promotes high availability configurations. The second step to distributed storage is separating the cache from the controller. Processing power and cache are the two main tools that RAID companies have for improving storage performance. Most RAID controllers on the market have a limited amount of cache. Therefore, distributing their processing capability may not be enough to offer enterprise performance. In order to meet enterprise performance requirements, companies like Texas Memory Systems offer external storage cache. These external storage cache systems can be flexibly combined with distributed RAID controllers to provide performance where performance is needed.

But how do they work? At its core, an external storage cache is a non-volatile array of fast RAM that interfaces with other storage devices through multiple high-performance interconnects, such as Fibre Channel links. The external storage cache can be flexibly attached to either server(s), switch(es), or storage. A storage device connecting to the external storage cache thinks it is talking to a server. A server talking to the external storage cache thinks it is talking to storage. In other words, the external storage cache is transparent to the storage network. Like the cache in monolithic RAID systems, the cache is battery backed. When external power fails, the unit will run fully functional off of its batteries. After a set amount of time, if power is not yet restored, the system will dump data from the cache to the attached RAID systems. Some external cache solutions will also dump the cache to internal disk drives as an added layer of data protection.

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Properly designed, an external storage cache will automatically discover storage targets on the network and present these to an administrator as candidates for caching. The administrator then maps the LUNs or storage devices that need caching to the external storage cache. In this way, one external storage cache can be used to accelerate one LUN, multiple LUNs, one RAID, or multiple RAIDs.

External cache systems generally support standard caching modes, such as write-through, write-back, and read-ahead. The most sophisticated of the external cache systems also offer the ability to flexibly configure the cache line size to match the size of incoming data traffic and to control the rate at which data is flushed to attached RAID systems.

External storage cache systems accelerate data access by providing a high-speed data pipeline. By temporarily writing data to fast RAM instead of conventional rotating disks, large cache systems drastically increase burst bandwidth and I/O rates, ensuring that existing storage systems will survive under immediate pressure. Smart caching algorithms "pre-fetch" data that users are likely to access, which increases overall performance by enabling applications to read the most popular data directly from cache.

In addition, some external storage cache systems accelerate the transfer rate of the RAID itself through a process called "data smoothing." Random data access behavior is extremely detrimental to conventional hard-disk performance because rotating drives are optimized for sequential data reads and writes. In the diagram, a typical low-cache RAID system is hit with a random data pattern. The RAID provides sustained data rates of 20Mbytes/second to the application. The same test with an external storage cache installed in-between provides dramatically different results. The same data pattern to the external cache system results in 150Mbytes/second of data throughput to the application. Part of this performance improvement is the result of write and read caching on the external storage cache. One of the intriguing results of this test, however, is that data is moving at 70Mbytes/second between the external storage cache and the RAID. The reason is that the external storage cache is smoothing the data by converting random data patterns to nearly sequential data patterns. Because the RAID is responding faster, the external cache is more effective, and as a result the application is responding faster to the user.

In addition to performance, external storage cache systems provide a number of other benefits:

Flexibility. External storage cache units can be applied where performance is needed. The units can be used to accelerate a single LUN or multiple RAID units. If more performance is needed for another application, it is easy to add another external storage cache to the network. Since it is not tied to the proprietary systems of a certain storage manufacturer, external cache systems typically have a superior array of configuration options.

Scalability. Internal cache locks a datacenter into finite and usually small maximum capacity. Many users have been forced to upgrade entire storage environments simply to have access to larger amounts of internal cache. External storage cache units have superior capacity densities compared to internal cache systems and easily scale by adding units for extra performance and capacity.

Broader use. Typical storage cache is restricted to use with one storage subsystem, but an external cache can be attached to multiple storage devices simultaneously, either directly or through a switched fabric. Depending on the features of the external cache, a user can carve up the cache for use by certain systems and specify specific cache behaviors per storage system. Companies can reuse a single external disk cache across the entire networked storage infrastructure.

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Cost effectiveness. Beyond the inherent value associated with its performance and flexibility, external storage cache systems offer immediate savings. It delays or avoids expensive upgrades and data migration by accelerating existing storage and extending its useful life.

As applications and data volumes continue to expand storage environments, most organizations face tough choices: data migration challenges, matching capacity and performance requirements, and the prospect of massive hardware upgrades. Independently scaling storage performance by adding external cache devices to the storage network alleviates these pains. Using a cache solution is beneficial from a return on investment and from management perspectives.

Cache-based systems already have a permanent place in modern IT environments. As business applications become more demanding, these external storage cache systems will become more necessary. Monolithic RAID configurations are not scalable to meet enterprise application needs and are too expensive for the SMB looking for performance. Combining distributed RAID controllers with external storage cache offer high capacity and high performance, while limiting overall cost.

Cache vendors, solid-state disk manufacturers and IT integrators can answer questions about external cache systems and how to best implement them in a given environment. An increasing number of RAID vendors are supporting external storage cache with their products. Contact your RAID vendor to find out how you can improve your storage ROI.

www.texasmemorysystems.com

Woody Hutsell is executive vice president at Texas Memory Systems (Houston, TX)