A hybrid flash array is a solid-state storage system that contains a mix of flash memory drives and hard disk drives (HDDs). Hybrid flash arrays employ form factors and electrical interfaces that are compatible with common HDD bays. This enables a gradual replacement of spinning hard disks with solid-state drives (SSDs) if desired.
Flash memory, which has no moving parts, is a type of nonvolatile memory that can be erased and reprogrammed in units of memory called blocks. It is a variation of electronically erasable programmable read-only memory (EEPROM), which got its name because its memory blocks can be erased in a single action or flash. A flash array can transfer data to and from SSDs much faster than electromechanical disk drives.
Flash is more expensive than spinning disk, but the development of multilevel cell (MLC) flash has lowered the price of SSDs. MLC flash is slower and less durable than single-level cell (SLC) flash, but companies have developed software that improves its wear levels to the point where MLC is acceptable for enterprise applications. SLC flash remains the choice for applications with the highest I/O requirements; however, there are hardly any still in use.
The benefits and drawbacks of a hybrid flash array
The difference between a hybrid flash array and an all-flash array is that hybrid storage arrays have some HDDs, whereas all-flash arrays have none. A hybrid storage array can feature a lot of flash, or just a little. Adding a small number of SSDs to a traditional array can improve its overall performance immensely, and it involves less of an upfront expense than an all-flash array. Frequently accessed hot data resides on flash, with colder data going to HDDs, improving the performance of the hot data without adding the cost of flash for data that does not require fast access.
HDDs are predictable and offer a low cost per gigabyte. However, they have a high cost per IOPS, use a lot of power and are slow. All-flash arrays have a low cost per IOPS, offer fast performance and use much less power than HDDs. However, all-flash arrays are more expensive on a per-gigabyte basis, and they present what many organizations perceive as an untenable upfront cost. Additionally, capacity claims are contingent upon added data reduction techniques, such as compression and data deduplication.
Hybrid storage arrays are the best of both worlds: They offer a relatively low cost per IOPS and gigabyte, as well as balanced cost for performance. Hybrid arrays meet the needs of most workloads because they make more IOPS available and reduce read latency.
Hybrid flash storage systems are right for any organization that needs faster access to stored data than spinning disk can provide. If all of an application's data does not need to be available to users at any given moment, then storing hot data on SSDs is a good idea. For this reason, many applications -- from collaboration and email applications to virtual desktop infrastructures -- benefit from added flash.
How do hybrid flash arrays affect enterprise storage?
There are hybrid storage arrays that ship with HDDs and SSDs in the same box. Alternatively, hybrid flash storage customers can attach flash drives directly to server PCI Express (PCIe) slots. The second option is usually more expensive, but delivers faster storage performance; storing hot data in PCIe flash reduces latency significantly.
Adding flash to a traditional HDD array is a simple way to improve performance, as filling the whole array with SSDs would be extremely expensive. In a shared storage environment, there's only a small subset of data that needs that kind of performance. Block-level dynamic tiering enables IT departments to target flash to those workloads that need improved performance.
If your workloads need peak performance -- as is the case with big data analytics and similar applications -- then all-flash arrays are the right choice. The raw capacity of all-flash arrays can seem small for their price, but most all-flash array vendors include deduplication, giving the arrays highly effective capacity rates.
Still, all-flash arrays can't match the capacity of hybrid and HDD options. With HDDs, customers can get a great deal of space for not a lot of money, which is ideal for file storage and backup and recovery. If your company has only enough money for one new array, but wants to keep capacity and improve performance, a hybrid storage array is the best option.
Hybrid flash storage performance
Hybrid arrays can decrease latency to 3 to 5 milliseconds; all-flash arrays can get that number down to a sub-millisecond. SSDs don't have pre-fetch problems, and all I/Os are cache reads, so there's no variability between a read and a seek. Still, most organizations can't afford to outfit their entire data center with SSDs, which is where hybrid flash arrays come in.
As the price of the array goes up, the cost of operating it goes down -- the more flash an array has, the less power it consumes. SSDs also take up less space, so organizations save on floor space and cooling, as well as power.
Considering cost per transaction -- rather than cost per IOPS or cost per gigabyte -- all flash is the cheapest option. Workloads that demand a lot from storage have a lower cost per transaction on all-flash arrays, whereas low transaction workloads cost less on hybrid storage arrays.
Users will notice a difference with all-flash arrays, especially in fundamentally high-latency environments, such as virtual desktop infrastructure (VDI) deployments. With a technology that already faces failure simply if users don't like it, adding some SSDs could be the difference between a successful VDI deployment and one that fails. That said, any applications can benefit from hybrid flash storage.
As of this writing, the major players in the hybrid flash array market are Dell EMC, Hewlett Packard Enterprise, Hitachi Vantara, IBM, Infinidat, NetApp, Tegile -- now part of Western Digital -- and Tintri. There are other, smaller vendors in the market, but enterprise hybrid storage array vendors usually choose from the bigger contenders.
Learn about the enterprise features and benefits of all-flash storage arrays and find out why more companies are choosing to adopt them.
Read an expert review of IBM's DS8870 Storage Array Platform and find out how the Easy Tier Server function helps with hybrid flash deployments.
Compare the features in the Fujitsu Eternus DX8900 S3, which supports hybrid flash configurations.