The Solid State Drives

Published Date: 02 Nov 2017

SSS, SSD, HDD

1. INTRODUCTION

A Solid State Storage (SSS) is a type of computer storage media, made from silicon microchips. A solid state drive (SSD) (also known as solid state disk) is an electronic storage drive built on solid state storage architecture (integrated circuits) to store data persistently. SSDs lack any moving mechanical components, unlike traditional hard disk drives (HDDs) that contain movable read/write heads and spinning disks. SSDs are rapidly becoming a popular alternative to hard disk drives as permanent storage, particularly in notebooks and PCs, because of their small size, faster read access, shock resistance , low power consumption as compared to hard disks drives (HDDs).

In this report after giving a brief introduction of the components of a SSD, we will discuss the results and observations of our experience while using this path breaking technology (SSD) as compared to HDD technology.

2. MAIN COMPONENTS OF AN SSD

A typical SSD consists of a printed circuit board (PCB) which contains an interface to connect to the computer, a SSD controller and a number of modules of memory chips [2]. Major

Most SSDs support the same host interface which is used in HDDs such as Serial Advanced Technology Attachment (SATA). Such a common interface guarantees backward compatibility, which avoids tremendous overhead for migrating existing HDD based systems to SSD based platforms [3].

2.2 Controller

The controller is an embedded microprocessor that executes firmware-level code and manages access to input/output (I/O) and read/write (R/W) operations between the SSD and host computer. Due to the presence of this controller only, a typical SSD is generally faster than a hard drive.

2.3 Memory

The primary memory component in an SSD is either a DRAM volatile memory or NAND flash non-volatile memory. The latter more commonly used nowadays [3].

2.3.1 Flash Memory Based SSDs

One finds a non-volatile NAND flash memory inside the SSDs. It is because they can retain data even in the absence of consistent power supply. This protects from data losses in power outage crisis and also lowers the cost as compared to DRAM based SSDs [5].

2.3.2 DRAM Based SSDs

SSDs based on volatile memory such as dynamic random access memory (DRAM) provide ultrafast data access (generally less than 10 microseconds). They are mainly used to speed up applications which otherwise would be slowed due to the latency of flash SSDs or traditional HDDs [5]. Either an internal battery or an external AC/DC adapter with backup storage systems is incorporated in these SSDs so as to provide data persistency in case of lack of power supply from the external sources.

2.4 Cache or Buffer

The DRAM cache used in a flash-based SSD is similar to the one

[5]

components of a SSD and their working are discussed in the

used in hard disk drives

. A directory of block placement and

following subsections:

2.1 Interface

wear leveling data is also kept in the cache while the drive is

operating. Data is not permanently stored in the cache.

2.5 Battery or Super Capacitor

The battery or the super capacitor is yet another important component in SSDs that help in maintaining data integrity, thereby turning the SSDs into higher performing ones. The data in the cache can be flushed to the drive when power is dropped; some may even hold power long enough to maintain data in the cache until power is resumed.

3. EXPERIMENTS

For the purpose of experimentation, we considered a Dell Inspiron

15 laptop having a 500 GB HDD (Momentus 7200.4 500 GB

7200 rpm SATA 3Gb/s) and another, a Lenovo U-310 Ultrabook which had a hybrid of HDD (500 GB) and SSD (SanDisk U100

24 GB SATA 3 Gb/s).

Both of them were assessed on a number of parameters like data transfer rate (read speed, write speed), access time, latency, cost, noise, vibration, weight, etc. We used Windows System Assessment Tool (WinSAT disk [15]) to get some hands on the details.

We could not include pure SSD ultrabooks due to their unavailability in most of the showrooms and moreover, their prices soaring high. However, we went to some of the showrooms and in one of them (HP), we were able to find a pure SSD ultrabook (128 GB), costing 1 lakh. We again ran the WinSAT command in front of them to check for some of the features.

Next section presents a overview of observations with inputs from the specs, results of WinSAT on the two laptops and some previous studies.

4. OBSERVATIONS (Solid State Drives vs. Hard Disk Drives)

4.1 Data Transfer Rate

The data transfers in SSDs occur in a flash as compared to the HDDs, which take several seconds depending upon the content being transferred. The transfer rate ranged between 100 MB/s and 400 MB/s, while in case of HDD based laptop, this went up to a maximum of 40 MB/s. In case of HDDs data transfer rate also depends upon rotational speed and also upon the track (reading from the outer tracks is faster due higher absolute head velocity relative to the disk).

4.2 Random Access Time

In SSDs, since data can be retrieved directly from various locations of the flash memory, access time is less than 100 µs [6]. On the other hand, in case of HDDs it is around 12ms owing to the movement of the heads and disk latency for rotation under the read/write head [7].

4.3 Read Latency Time

In SSDs, the read latency time is generally low because the data can be read directly from any location. Comparing with their HDD counterparts, it is much higher because read time is

different for every different seek, since the location of the data on the disk and the location of the read-head makes a difference.

4.4 Start-up Time

Startup in case of a laptop containing SSD is almost instantaneous as compared to a laptop having only HDD as the storage medium. This can be attributed to the absence of mechanical components in SSDs which otherwise require preparation; and disk spin which take up several seconds of startup.

4.5 Noise

Due to the absence of any moving part in an SSD, the SSD based laptop ran without any noise. However, the HDDs have moving parts (heads, actuator, and spindle motor) causing the HDD based laptop to make sound. Also, these noise levels vary from model to model. For example, HCL laptop produced more noise as compared to an HP one.

4.6 Vibration and shocks

Even though the SSD could not be touched but it can be safely concluded that they are resistant to shocks and vibrations once again due to the absence of moving parts. Meanwhile, the HDD would often vibrate and would at times, give shocks owing to the heads floating above rapidly rotating platters.

4.7 Form factor and weight

Solid state drives, are essentially semiconductor memory devices mounted on a circuit board, hence are small and light in weight. The SSD only laptop weighed about 1.6 kg as compared to an HDD only laptop (3.5 kg). However, there are other factors included as well causing an increased weight of the latter. There is no significant difference found in the size of these two drives due to the enclosure in which they are fitted in. HDDs typically have the same form factor but may be heavier. This applies for

3.5" drives, which typically weigh around 700 grams.

4.8 Cost per GB

One of the few drawbacks observed in the SSDs are their prices. For a 500GB capacity internal 2.5-inch drive, it is required to pay between Rs. 4000 to Rs. 6000. However, when it comes to SSDs, the same capacity SSD can easily cost up to Rs. 20000. That translates into 8 Rupees-per-GB for the HDD and 40

Rupees-per GB for the SSD. The prices go up in a similar way

for different capacities.

4.9 Power requirements

When both HDD only and SSD only laptops were put to use using the same applications, it was seen that the SSDs consumed less power as compared to HDDs (nearly half to a third less consumption). However, there are some high-performance DRAM SSDs which require as much power as HDDs, and must be connected to power even when the rest of the system is shut down [14]. The lowest-power HDDs (1.8" size) can use as little as

0.35 watts [16]. 2.5" drives typically use 2 to 5 watts. The highest-performance 3.5" drives uses up to about 20 watts.

4.10 Reliability and Life

The conclusions on the reliability of the SSDs come from what the manufacturers of SSDs claim and what other studies have shown. The SSDs have no moving parts to fail mechanically. Each block of a flash-based SSD can only be erased (and therefore written) a limited number of times before it fails. As a result, the controllers manage this limitation so that drives can last for many years under normal use [9].

HDDs have moving parts, and are subject to potential mechanical failures from the resulting wear and tear. The storage medium itself (magnetic platter) does not essentially degrade from read and write operations. According to a study performed by Carnegie Mellon University for both consumer and enterprise-grade HDDs, their average failure is of 6 years, and life expectancy is of 9 – 11 years [10].

5. LIMITATIONS

Unlike mechanical hard drives, current SSD technology suffers from a performance degradation phenomenon called write amplification, where the NAND cells show a measurable drop in performance, and will continue degrading throughout the life of the SSD. A technique called wear leveling is implemented to mitigate this effect, but due to the nature of the NAND chips, the drive will inevitably degrade at a noticeable rate. Also read performance does not change based on where data is stored on an SSD. Whereas in case of HDDs response times will increase if data from different areas of the platter must be accessed due to the need to seek each fragment.

6. CONCLUSION

From the above comparison we can conclude that most of the advantages of solid-state drives over traditional hard drives are due to their ability to access data completely electronically instead of electromechanically(in case of HDDs), resulting in superior transfer speeds and mechanical ruggedness. In case of SSDs, the flash memory controller and its firmware play a critical role in maintaining data integrity. One major cause of data loss in SSDs is firmware bugs, which rarely cause problems in HDDs. Also SSD failures can be catastrophic, with total data loss whereas HDDs allow much or all of their data to be recovered in case of a failure.

Seeing both the advantages and disadvantages of SSDs as compared to HDDs we can say that SSS and hard disk storage

are complementary to each other and not antagonistic. Hence it can be concluded that HDDs can be used where high capacity is the main requirement, whereas SSS can be used where high performance is the main requirement. Also since the overall volume of data is increasing rapidly, HDD storage is also not going away any time soon, but almost certainly the role of hard disk drives will evolve to accommodate the increased presence of SSDs in future storage solutions.