Bench Talk

On the left, the Hard Disk Drive. On the right, the Solid-State Drive Source: MacWorld.com

Non-volatile storage is a key component in any computer system. Traditionally a computer will contain a hard disk drive (HDD), a rotating disk with data magnetically written on it. Disk drives were invented over half a century ago and have since become small and cheap. Enter the new kid in town, the solid-state drive (SSD). Solid-state drives use non-volatile flash memory to write and read data at speeds far greater than an HDD. Although SSDs have existed for a couple of decades, only recently have they become cheap enough with enough storage space to become mainstream. SSDs are increasingly common in modern computers, especially in tablets and laptops. With this in mind, in what ways are SSDs better than HDDs, and is the difference significant enough to signal a complete revolution in the storage industry?

A quick note: enterprise storage applications have different, often more robust requirements to consider. Some numbers may be out of date as memory is a continually improving and every manufacturer has different specifications.

Table 1: Simplified comparisons of the two storage types.

The main draw of SSDs is their performance. When searching for or writing a file, a hard drive has to spin a disk and move a mechanical arm to the right spot on the disk. HDDs have different write speeds depending on whether they are writing large files sequentially or small files fragmented throughout the drive. Sequential writes are much quicker than fragmented writes because the disk has to spin to the correct spot while the head extends to that location. Solid-state drives, on the other hand, use NAND flash to store data, which happens relatively instantaneously. HDDs normally have a data transfer rate of around 200 megabytes per second (MB/s) while SSDs have been shown to transfer data closer to 500 MB/s.

At the time of writing, SSDs are more expensive per unit storage than their mechanical counterparts. This is due to a mix of high manufacturing costs and the supply/demand situation. When a manufacturer puts an SSD in their computer, that SSD is a significant percentage of the total cost of the machine, which so far has caused computer companies to be slow in adopting SSDs. A 250GB HDD only costs about $10, barely affecting the computer’s retail cost. A 250GB Samsung 850 EVO, a reasonable amount of storage for a laptop, costs about $90-$100. A few years ago, the same amount of storage could cost you $500, which is more expensive than many Chromebooks using SSDs today. So SSD prices are declining, and higher-storage drives are coming out, but they are far from being comparable to HDDs on a price scale.

A small SSD now costs about as much as a medium-sized HDD, a trend SSD supporters can be excited about.

Long-term storage is a must for regular data applications. Nobody likes losing data because of an internal error, especially when the glitchy drive is your backup. Studying the lifetimes of hard drives is sort of a favorite subject with tech websites, so there are a few sources to pull from when comparing the two formats. BackBlaze found that 90% of HDDs survive for three years before they start to have problems; after three years, the failure rate per year goes up to around 12%. Tech Report tested SSD lifespans. However, instead of keeping track of the time it took for the drives to die, Tech Report ran six different drives continuously for 18 months and logged the amount of data written until the drives were rendered unusable. The sturdiest drive started reallocating sectors before hitting one petabyte (PB or 1000TB) but kept writing data until it had written 2 PBs when it failed. The data gathered by this study also logged the performance of each drive as it started failing, so if that interests you, check out the original article. These drives were purposely pushed to their limits for 24 hours every day during that time. In reality, the average consumer might write a couple of TBs every year. The study concludes that your SSD will probably survive longer than the rest of your computer, beating out the HDD’s 12% failure rate for every year after the third.

A chart from Tech Report's SSD Lifetime testing showing reallocated sectors piling up over many writes. Source: techreport.com

Power is an oft-forgotten but still important consideration. In large data centers, the efficiency of a drive can determine not only power usage but heat levels in the facility. Power consumption depends mainly on the particular drive and how much work it is doing. Tom’s Hardware did tests on different drives a few years ago showing that the most efficient SSDs ran at an average of 1 watt (W) while the most efficient HDDs used 4.8 W while writing at maximum throughput. Consumers should also consider power efficiency when buying a new laptop, as HDDs are louder (due to both mechanical drive noise and fan noise from the heat generated) and get hot sooner than SSDs.

When you are finished using a drive and need to dispose of it, getting rid of all of the data on it is important, whether you work for the government or you are simply someone who would rather not be a victim of identity theft. Studies in SSD erasure methods have led some to question how secure your data is, and whether deleting all of your files means that data is gone for good. Instead of actually erasing the data, when a file is deleted an SSD sometimes will merely make that sector unreadable through normal methods. The flash file system will not be able to read a deleted piece of data, but a hacker with the right tools can still find your deleted files. SSDs use wear-leveling to keep the storage cells fresh (preventing a certain sector from being written too often and wearing out), which for wiping purposes means that not every cell of data may be cleaned when you delete a file. HDDs, on the other hand, can be degaussed. Degaussing involves removing the magnetic field on the disc, which not only removes your data but also wipes away manufacturer formatting, ensuring that the hard drive will be completely inoperable.

Considering the above, SSDs are the storage format of the near future. SSDs are more expensive than HDDs, but they will only continue to decrease in price. Once SSDs are cheaper, more consumers and computer manufacturers will agree that the speed, lifespan, and efficiency of the SSD make for a more enjoyable personal computing experience. As for company workstations, servers, and data centers, it will probably be longer before the security issues with SSDs are handled sufficiently to allow a complete switch. Of course, HDDs will remain for archival purposes just as tapes are still kept around, but soon most people will have solid-state storage in their computers whether they realize it or not.