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Understand Your Options Before Choosing a Flash Storage Solution

A new trend for server storage is the implementation of flash memory technology to improve performance. This article offers an overview of how it’s implemented.

Flash Module Technology

The underlying technology for this type of storage is the flash memory module. The three classes of flash modules are:

  • Single-level cell (SLC)
  • Enterprise multi-level cell (eMLC)
  • Multi-level cell (MLC)

Data written to flash modules is persistent and doesn’t require constant application of electrical power like older solid-state storage technologies. Unlike traditional computer memory and disk drives, the data bits inside a flash memory module can sustain a limited number of write operations. The total number of supported write operations defines the module’s useful life. For example SLC supports about 100,000 writes; eMLC approximately 20,000 to 30,000; and MLC about 3,000 to 10,000 writes.

Flash memory module pricing is related to the number of write cycles it supports, so SLC modules are the most expensive and MLC the least. SLC and eMLC modules are classified as enterprise grade, while MLC modules are considered consumer grade. To select the appropriate type and avoid premature module failure, you’ll need to understand your application’s disk write trends. Note that read operations don’t affect the useful life of flash modules.


Flash memory modules are available in various configurations that make them useable with computers. The most common designs include:

Solid state hybrid drives (SSHDs) are based on spinning hard drives augmented with a flash memory module that’s packaged within the standard hard drive carrier and is typically around 8GB in size. This design can provide a significant boost in performance and offers a simple, direct replacement for standard disk drives.

Solid state drives (SSDs) are built entirely from flash memory modules with no spinning disk platters. Commons sizes range from 64GB to 512GB. SSDs are generally packaged in traditional 2.5-inch and 3.5-inch disk drive carrier form factors. This allows them to directly plug into server disk bays. Some SSDs, especially those for laptop computers, are custom devices that look more like a memory module.

All flash arrays (AFAs) package high densities of flash memory and custom controllers into rack-mounted drawers. Useable capacity is typically in the 10 to 20 TB range. AFAs generally provide higher performance along with RAID data protection and the ability to connect to multiple servers.


Three primary types of connections are used to implement flash-based storage with computer systems: Disk bay, PCIe and networked. SSHD and SSD devices plug directly into existing disk bays, which are controlled by SAS or SATA interfaces. This will generally be fine for a few SSD devices. However, connecting many SSDs to a single SAS/SATA controller, can result in less than ideal overall throughput. Some newer SAS controllers are designed to perform better with SSDs. These controllers have larger caches and improved processing capabilities. PCIe-based flash memory features an adapter card that plugs directly into a PCIe adapter slot in a server. These cards often utilize 1.8-inch SSD drives or have flash modules directly mounted to them. In either case, you’ll generally get higher performance with the PCIe card than SAS/SATA-connected SSHDs or SSDs. Networked flash solutions typically use FC or InfiniBand interfaces. Network connections are mostly used with AFA rack mount systems. The connections can be direct from storage to server or make use of an intermediary switch that facilitates multiple-host support.

Usage Considerations

When implementing flash, the primary concern is the data write rate for your applications. If an application is extremely write intensive, you may prematurely wear out flash-based storage. You’ll also need to consider the total amount of flash storage that you’ll require. When using flash, you can utilize close to 100 percent of the installed capacity. This can reduce the total flash capacity needed. With traditional disk storage solutions, users often restrict use to 20 to 50 percent of the total capacity due to read/write head contention. Also, if you’re using SSHD/SSD or PCIe cards, you’ll lose some capacity if you decide to implement RAID protection. AFA systems often include integrated RAID controllers and AFA specifications will frequently state useable capacity. If all of your data won’t fit onto the flash devices, you’ll need to figure out how to direct active (hot) data onto the flash device and the remainder onto traditional disk storage. This can be simplified with auto-tiering software.

If your total data is 10TB or less and your access pattern is mostly reads, one way to quickly implement flash is using OS-based mirroring. You can simply add the flash storage to your system as a mirrored copy. Then, read requests typically sent to both traditional and flash storage will receive a very fast response from the flash storage. Combining the two in this manner eliminates the cost of having to use expensive flash storage for both copies of the data.

Product Selection

When preparing to select a flash-based solution, choose the module type—SLC, eMLC or MLC—that matches your write activity level. When exploring the options, it also helps to know your latency requirements and the total number of I/O operations required for your workload. You can then consider other features like overall capacity, redundancy, number of connected servers, and whether you’ll need auto-tiering software.

Speed Things Up

Flash storage can greatly enhance the performance of many applications, especially when reduced latency is important. With many products to choose from, implementing flash is a possibility for most servers.

Charlie Cler supports customers in a solutions-architect role at Forsythe Technology Inc. He can be reached at

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