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Why do Flash Drives with the same Capacity have Different Costs?

Over the last few years, flash drives or solid-state drives (SSDs), as they are commonly known, have become very popular and for all the right reasons. First and foremost, it offers exponentially higher data input/output per second (IOPS) and much better performance (access time and latency) than the older spinning disks or hard disk drives (HDDs), almost to the tune of 20x in some cases. They consume less power, generate less heat, and cause lower acoustical noise—no moving part, so less physical wear, and tear. Most importantly, the price point is trending downwards, making them much more affordable, reflecting a continuous evolution of the technology and economies of scale.

Now, all that is good, but do you ever wonder why there are different costs for the same flash drive capacity with a similar interface (SAS or SATA) from the same manufacturer or why a 3.2TB flash drive costs more than a 3.8TB flash drive?

When I ask my customers which flash drive they want while building a server or a storage solution, they are often unsure. Sometimes they opt to go with the least expensive one without understanding how it may impact their client's total cost of ownership (TCO).

One of the key factors contributing to a flash drive's varying cost is its endurance, or in simple words, how long the drive will last before one needs to replace it.

Writing to a Flash Drive

To understand flash endurance, we need to know how a flash drive writes. At a very high level, we should understand that when a write instruction comes to the flash drive, the drive cannot directly write on to an existing block/page/cell that already contains data. It copies the existing data with the changed data on a new empty block/page/cell. Once done, it erases the old data, and that block/page/cell becomes available for a new write.

Here's an everyday example to help you better understand the process. Imagine that you have a piece of paper that you can write on with a pencil and fill the entire page with content. The next time you want to write something on that same paper, you have to erase it first and then write on it, and the process continues. There will come a time when you cannot erase and re-write on the page anymore as it has worn out. You have to discard it and start with a new piece of paper.

When data gets written on to a flash cell, which is the smallest unit of a flash medium, the cell holds an electric charge (data) in a transistor with insulation around it. This insulation wears out with increasing write/erase cycles, and eventually, the cell cannot be used anymore.

How many write/erase cycles a flash cell can sustain depends on the flash technology used in the drive. The most common flash technologies are Single-Level Cell (SLC), Multi-Level Cell (MLC), and Triple Level Cell (TLC). The maximum number of write/erase cycles an SLC flash cell can sustain is about 100,000. However, a TLC flash cell can sustain about 3,000 write/erase cycles.

Also, to enhance these endurance levels, various manufactures of disk and storage use additional technologies such as Wear-Leveling, Over-Provisioning, Bad-Block algorithm, to name a few.

Once all encompassed, manufacturers publish a metric to measure a flash drive's life in terms of "Drive Writes Per Day" (DWPD) measured over a set period, typically 5 to 7 years.

Usually, the higher the DWPD rating, the more expensive the drive is. Hence you see flash drives with the same capacity from the same manufacturer having different price points.

Now let's consider the example of a 1.92TB flash drive with 1 DWPD measured over five years. This drive can sustain a full capacity (1.92TB) write per day for up to five years. If one writes more data than 1.92TB per day on that drive, it likely will not last up to 5 years, which brings us to my next point about what to do before selecting the flash drive.

Before selecting the flash drive, we should try to understand the environment's read/write characteristics. For a more read-intensive workload (e.g., 70/30 read/write ratio), a flash drive with a lower DWPD, for example, 1 to 3, should be okay, whereas for a more write-intensive workload (e.g.,  20/80 read/write ratio), it's better to go with a 5 to 10 DWPD flash drive. Otherwise, we may end up causing customer grievances by selling a low DWPD drive for a write-heavy environment or overselling a high DWPD drive in a read-intensive environment.

If you have any question or want to learn more about flash storage, please contact us.

About the Author

Sr. Technical Solutions Engineer – IBM Power Systems and Storage Data Center Solutions & Professional Services - - Saad Islam has developed his expertise in data center infrastructure solutions through his keenness in going deep and learning the inner detail of technology. With his “do it right” attitude and passion for technology Saad loves to help his business partners understand their options and choose the right solution for their clients’ business needs.

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