Part 3 – The advantages and disadvantages of HDD and SSD
In the first and second articles, we looked at the hard disk drive (HDD) and the solid-state drive (SSD) respectively. In this third and final article of the series, we will be looking at how the two storage technologies compare.
Below is a list of attributes that we will be comparing the two devices on:
2. Power Consumption
4. Data Persistence
The HDD is slower than the SSD when it comes to reading and writing data. This is because the HDD needs to spin the platters and wait for the read/write heads to reach their respective spots. Only then, are they able to read or write data bit by bit based on the instruction received from the system. In the event of severe file or space fragmentation and heavy disk utilisation, the HDD will need to spin its platters continuously while the read/write heads are moved repeatedly across the surfaces just to read or write data. This can cause the system to become unresponsive.
On the other hand, the SSD has no moving parts. Data is stored in memory cells within the NAND flash chips. Access to these memory cells is virtually instantaneous from the user perspective.
Now, you might be wondering how is it that SSDs can be any faster, when USB flash drives are slow, given that they use the same kind of flash technology. Yes, you are right. Speed was a problem especially with earlier USB flash drives. Taking a reference from this article1, a flash drive could only achieve a maximum write speed of somewhere between 40MB/s to 60MB/s at best. The reason is simple. To achieve the pricing that is accessible to the mass market, the parts used in USB flash drives are of lower specifications. That means the memory controller is much simpler in its operations and the memory chips used are slower.
But that is not the case for SSDs.
In order to achieve the high read and write speed needed in SSDs, the drive comes with an advanced controller that does more than reading and writing data to the memory cells. It does some basic wear levelling as well. On the SSD, parallelism is fully utilised. The memory controller is connected to the memory chip through a series of parallel communication paths, allowing the memory controller to read and write data across multiple pages concurrently.
In terms of power consumption, the HDD has a higher power draw as compared to the SSD. And that is even during idle state. Citing the Western Digital Blue series2 of HDD as an example, the lowest average idle power consumption you can find is 2.5 watt and during read/write operation, the highest average power consumption is 6.8 watt.
On the other hand, citing the Samsung EVO 860 SSD3 as an example, it has an average idle power consumption of 50 milliwatt and during read/write operation, the highest average power consumption is 3 watt.
That means for every HDD you have in your system, you could potentially run two SSDs.
A HDD is more sensitive to physical trauma than a SSD due to the moving parts it contains. The most sensitive component is the read/write head. It is designed to fly over the platter surfaces with clearance of only a few nanometers to read and write data. This would mean that, if the HDD is in operation and the system suffers some form of shaking, it may cause the read/write heads to crash into the platters.
And even if the HDD is powered off, it is not completely safe from impact. Although modern HDD will park the read/write heads in a region where no data are stored, a moderate-strong impact may knock the read/write heads out of position or alignment and cause them to crash into the platter. Such an impact may also cause the glass and ceramic-based platters to shatter.
On the other hand, the SSD is more durable simply because it does not have any moving parts. This means that any moderate shock or impact will not affect its ability to keep your data safely stored, thus making the SSD a more suitable storage device for mobile phones and laptops.
Data does not last forever in both the HDD and SSD after the power is cut off. Their ability to store data is dependent on the environment they are kept in.
For a HDD, the magnetic fields on the platter(s) can and will degrade over time. Even then, they can hold data for up to seven or eight years if the drive is kept in a temperature of around twenty-five degrees Celsius with low humidity, as cited in this article4. Any increase in the temperature or humidity can cause the magnetic fields on the platter to degrade faster, causing data loss or corruption.
On the other hand, the memory cells in SSDs can leak stored electrons over time. Two thin layers of oxide insulators are all that separate the memory storage region from the rest of the memory cell. Every time a memory cell undergoes the erase-and-write cycle (also known as program-erase cycle), the insulator wears down because a higher voltage is needed to erase a cell before it can be programmed again. And with the recent advancement in the production of flash memory, the insulator layers are only getting thinner, which will further affect the lifespan of the insulator.
This insulator degradation particularly affects SSDs with triple-level (TLC) or quad-level (QLC) memory cells. As their names suggest, these types of memory cells can store three and four bits of data respectively. In order to achieve that, they rely on different, predefined voltage range for each bit.
Let us use the TLC as an example.
To store the binary value of “001”, the SSD could use a voltage range of 0.10V to 0.12V to represent it. Then to store a value of “010”, it could use a voltage range of 0.125V to 0.145V to represent it. In the event of any electron leakage for the latter value, the voltage level of the stored charge will drop. For example, it could go down from 0.145V to 0.124V. In such a scenario, there would be data corruption, as the SSD is unable to determine the original value.
Between the HDD and the SSD, the former remains the winner on the price front and will remain so for the foreseeable future. Today, a 1TB internal HDD costs between US$44 to US$80 while a similarly-sized SSD costs around US$150.
One reason for the price difference is that the HDD uses an older, more established technology that allows manufacturers to offer even bigger capacities while improving performance in the same form factor. And even though there are new developments in how data is stored on the platters (eg. making the magnetic field vertical instead of horizontal), the core technology for storing, reading and writing
data remains largely the same.
On the other hand, SSDs are generally more expensive due to its manufacturing cost. NAND flash is an integrated circuit built using similar processes as the CPU. In order to manufacture them, manufacturers need to acquire expensive silicon wafers, build multi-billion dollars fabrication plants (if not outsource to a third-party fabrication company), and spend a significant amount of money, time and human talent to research, design and develop it. The fabrication plants are also very sensitive to industrial incidents such as power outage. A single power outage lasting only minutes could destroy thousands of NAND flash wafers5 and damage manufacturing equipment6 . Therefore, such costs have to be amortised and is baked into the price of a SSD.
The storage capacities of both HDD and SSD are tied closely to their prices. Higher capacity translates to higher price. The difference between the two lies in how much does the price increases with respect to the increased storage capacity. Generally, if you are looking for higher storage capacity, HDD would be the better choice.
The storage capacity for commercial HDD can go up to 12TB and 16TB for enterprise-grade HDDs, as of this writing. For most situations, a 4TB HDD would offer plenty of storage space for us to storage large files such as videos and images while remaining relatively affordable at around US$200.
On the other hand, a 4TB SSD would cost US$700 on average and those of even higher capacity would come at prohibitively high prices. Although smaller SSD capacities of around 128GB-512GB cost less, they are still more expensive than a similarly-sized HDD and may not suitable for storing large number of large files such as videos and images.
Which is more suitable for me?
Both the HDD and SSD have their place in any given system. It all depends on what is important to you or your company.
Below is a table that summarises the advantages and disadvantages of HDD and SSD.
Now that we know the difference between HDD and SSD, choosing between the two should be a relatively much easier and straightforward process. In some cases, it may even be beneficial for both storage technologies to co-exist in a given system.