Hybrid and Solid State Drives

In the past few years we’ve seen some long overdue improvements in data storage technologies. Every other aspect of computer technology has rapidly advanced: multi-core CPUs, faster motherboard buses and faster and inexpensive memory has resulted in hard disk storage becoming the bottleneck to greater system performance. Fortunately, hybrid drives (SSHDs) and solid state drives (SSDs) have greatly improved data storage performance.

Solid State Hybrid Drives are traditional SATA mechanical drives (rotating disks with ganged read/write heads) that include a smaller amount of solid state memory, typically 8GB-24GB. The memory that is used in SSHDs is the same type of memory, NAND flash memory, that is used in SSDs, but how and when it is used to store data is quite different from SSD storage. SSHDs operate in one of two modes: Self-Optimized and Host-Optimized.

In Self-Optimized mode the SSHD works independently from the operating system or device drives to make all decisions related to identifying data that will be stored in NAND flash memory. This results in a storage product that appears and operates to the host system exactly as a traditional hard drive would. 

In Host-Optimized mode the SSHD uses extended SATA commands. Using this feature, decisions about which data is stored in the NAND flash memory comes from the host operating system, device drivers, host software or a combination of thereof.

Performance of SSHDs over traditional HDDs is mixed. Random read/write performance of SSHDs is no better than HDDs, as would be expected. The major performance improvement comes in application startup and shutdown. 

Solid State Drives are, strictly speaking, not ‘drives’, since there is no disks to be driven. Instead, it’s more accurate to refer to them as large-scale flash drives. In fact, they use the same type of memory used in USB Flash Drives. These types of drives excel in providing faster data access because there is virtually no latency in accessing the data elements within the SSD. Traditional HDDs incur latency in two areas: rotational latency and butterfly read/write latency. Rotational latency is the result of the disk system ‘waiting’ for the disk to rotate the platter to the location on the disk where the desired data is stored. Butterfly read/write latency is the result of the time it takes for the ganged read/write head to move from its current position to the location where the data is stored. Typical combined rotational and butterfly latency rates are in the range of 4ms per read/write. While this may seem insignificant, multiplying this value by the number of times this operation must be performed per file, in excess of 10 times, results in significant delays, particularly when loading applications consisting of dozens of separate files. SSDs have no latency since their architecture is optimized for truly ‘random’ access. SSDs also consume less power, typically 1/20th the power of HDDs. 

Since SSDs can access data anywhere in its memory array with equal speed, there is no need to defragment data on SSDs. Defragmenting SSDs, while possible, will not result in improved performance and WILL result in significantly reducing the life of SSDs. Windows 7 and Windows 8 will detect the SSD and disable automatic defragmentation when installed on the SSD. Additionally, SSD performance may degrade over time when used on operating systems that do not support the SSD “trim” command.  Operating systems typically ‘delete’ data by only erasing the first byte of data containing the beginning of file mark. On SSD, when all of the SSD storage has been written to, the SSD must then completely erase the memory locations prior to writing new data to those locations. The erase operating adds additional overhead to the write process. Newer Windows operating systems, Windows Vista and later, support the “trim” function. Also, newer SSDs have incorporated their own “garbage collection” system that automatically performs the trim function.

While, currently, SSDs are considerably more expensive than traditional HDDs and SSHDs, they still represent a viable option for improving the performance of servers and workstations. Using SSDs for laptops can extend battery life and dramatically improve performance.  

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