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Guide to Flash Storage Memory

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Solid state drive vendors initially introduced flash storage memory and SSD’s to support high-end applications and high-performance storage tiers. Deployments have rapidly expanded over the last few years as flash/SSD performance increases – with better SSD IOPS benchmarks – and price decreases.

In 2019, the SSD vs HDD debate is largely over: even though hard drives still comprise 80% of most data centers media, and tape is still in the picture, flash SSD deployments are growing with extraordinary speed. As the enterprise storage market grows, flash seems to grow even faster.

What Is Flash Memory and SSD?

Let’s define some important terms early on: flash and SSD are closely related, but are not synonymous.

  • Flash: Flash memory is non-volatile computer memory for persistent storage and data transfers between PCs and digital devices. Flash’s high-speed programmable memory performs fast random I/O operations.
  • SSDs: Solid-state drives are not by definition flash devices. SSDs are not memory; they are media. In fact, the first SSDs used volatile DRAM memory. Today most SSDs use non-volatile NAND flash memory for persistent data storage. Although much of the industry use the terms interchangeably, that is changing with the introduction of SCM (storage class memory). SCM is not a flash technology.

How do SSDs Work?

SSDs read and write data to a silicon layer of interconnected flash memory chips. Multiple chip layers enable different densities. The SSD’s two most important components are a flash controller and flash memory chips, usually NAND flash. The controller adds storage intelligence to the drive.

HDDs are built from multiple moving parts, which over time do wear out and stop working. In contrast, SSDs with their integrated circuits have no moving components. SSDS are also more resistant to physical shock, run silently with lower energy costs, and offer higher performance and lower latency than HDDs.

Power and cooling costs are significantly lower than HDD’s because SSD’s have no moving parts, so generate far less heat than a disk drive’s spinning disk. And because SSDs perform much faster, they are idle more often than HDDs, so use minimal energy.

SSDs are also faster than HDDs and more durable if they have a consistent power source. Having said that, SSD storage is more dependent on being connected to a consistent power source than HDD’s or tape. Short power interruptions are not a major issue, as SSDs are built with floating gate transistors (FGRs) to retain an electrical charge. However, a long disconnection will spring data leaks. Since powerless SSDs will not retain integrity indefinitely, they are not suitable for archiving.

Types of SSDs

  • SATA and SAS are interfaces between storage and the host. To be sure, some see a SAS vs SATA debate, but both have their place.Traditionally they were developed for hard drives, and still act in that capacity. SSDs were developed to fit into the same interfaces, so users could more easily switch storage arrays to share SSDs and hard drives. Although SaaS is considerably faster than SATA thanks to parallel processing, both interfaces are slower than newer technologies like NAND.
  • NAND is non-volatile flash storage whose chip formats fit SSD media.  fits into different formats including SSDs. NAND flash is the most common flash in SSDs. Newer 3D NAND stacks cell layers vertically, which dramatically improves density and endurance, lowers power consumption, and accelerates read/write times.
  • NOR flash memory executes in place without having to copy into RAM first. This gives it very high transfer speeds. But it has fewer gates than NAND and correspondingly lower write and erase speeds.
  • NVMe is an interface that accesses flash storage using a PCIe bus. This enables the interface to support many thousands of parallel queues, each supporting many more thousands of simultaneous commands. Using NVMe in dual-port systems provides scale-up and scale-out capabilities in enterprise SSD systems. NVMe-oF (NVMe-over Fabric) is a host-side interface that extends NVMe functions over fiber Channel fabric or remote direct memory access (RDMA). 
  • SCM (storage class memory) is a non-flash, persistent memory technology that incorporates storage. It truly is a disruptive technology. Like DRAM, SCM connects to memory slots in a motherboard. It is slightly slower but persistent. Its read/write technology is many times faster than flash, and is not subject to wear on its storage cells.

SSD Lifespan

New technologies like FGR and wear leveling have lowered SSD failure rates. However, SSD architectures affect lifespans, which differ by how many bits NAND cells hold: Single Level Cell (SLC), Multi-Level Cell (MLC) and Triple Level Cell (TLC). As you might have guessed, SLC saves one bit, MLC saves two, and TLC saves three in a cell.

For more information about flash types, see: SLC vs MLC vs TLC NAND Flash

The issue is wear: the damage done to an SSD cell over time. Reads do not appear to impact durability, but writes and deletions do. The more bits the cell stores, the less damage that writes cause. Important note however: larger cells improve cell durability but do not improve latency. TLC read latency is 4x slower than SLC, and write latency is worse by a factor of 6. Erase latencies are also slower on TLC.

Wear-leveling increases durability on all chip levels by spreading write operations among cells to balance overall wear and SSD durability. Although SSDs do not have the durability to archive data long-term, they are expected to last at least 2-3 years without data leaks or component failures.

Flash and SSD Speeds

Flash and SSD speeds have increased many times over the years. In 2018, Samsung and Toshiba both introduced 30.72TB SSDs using a 2.5” form factor with 3.5” thickness. SAS interfaces are faster than SATA and slower than NVMe, but much less expensive than the latter. Samsung also offered an M.2 SSD with speeds of 3500MB/s.

SSD Transfer Rate

ssd read write speeds

Examples of SSD write speeds as measured in megabytes/per second. This illustrates that while SSD is fast, some SSDs are far faster than others.

It is difficult to measure SSD performance as a class since different SSDs offer very different performance, transfer, and latency metrics. In general, SSD access times are a fraction of hard drive access metrics, can deliver IOPs from 6000 to 1 million, and consume about 5 watts of energy as opposed to HDD’s 6-15 watts.

Accelerated application performance is the primary benefit of SSDs. Flash demonstrates 20 times performance of HDD’s, making them particularly beneficial for I/O-intensive applications like high transactional databases, VDI, or big data analytics storage. When SCM becomes commercially available in about 12 months, this processing speed will increase enormously. SSD and Flash in Enterprise Storage Environments goes into more detail.

Are SSDs worth it?

In the data center, flash arrays are steadily growing thanks to lower prices and added storage intelligence. Decreasing SSD price trends improve the economics of flash-based storage in the data center, particularly for storage area networks. SANs are an expensive purchase, and IT has resisted spending even more money by moving from HDDs to SSDs. However, as the prices fall and performance pulls far ahead of hard disk drives, more data centers are buying all-flash SANs for very high performance at an acceptable price.

Ultimately the “best” flash storage is what is best for a business: what’s best for a web streaming enterprise is not for a mid-sized stockbroker. Look here for a list of the best M.2, SATA, and PCIe SSDs on the market today for different business needs: “Best SSD Buying Guide: Best and Fastest SSDs.”  For flash arrays, read “Best Flash Storage: Top 10 All-Flash Storage Array Vendors.”

 

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