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SOLID STATE STORAGE - THE

TECHNOLOGY, THE TECHNICAL

BREAKTHROUGHS AND THE FUTURE.

Presentation Report

MARCH 2014

Adedunmola Adenike Afolayan – M00440811 27/03/2014

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Table of Contents

Abstract ..................................................................................................................... 4

Introduction ................................................................................................................ 4

The History .................................................................................................................5

Hard Disk Drives ................................................................................................... 5

Solid State Disks (SSD) ........................................................................................ 6

All about SSS ............................................................................................................. 6

SSS Form Factor ....................................................................................................8

Solid State Drive …………………………….………………………..... 8

Solid state Card …………………………………………..……………. 9

Solid state Module ……………………………..…………………..….. 9

Components of SSD ...................................................................................................... 9

Memory………………………………………………………………….…...…. 9

Controller …………………………………………………………………..….. 10

Wear Leveling …………………………………………………………...10

Garbage Collection …………………………………………….…………12

Address Mapping Technology ………………………..……….…………12

Types of Storage State System …. ................................................................................... 13

Flash memory based ……………………………………..…………….……….…. 13

NAND Flash ……………………………………………………..……... 13

NOR Flash ……………………………………………………….………… 13

Ram based …………………………………………………………………….…… 14

Types of NAND Flash .......................................................................................................... 14

Single level cell …………………………………………………………………..… 14

Multi-level cell ………………………………………………………………….…... 14

Triple level cell…………………………………………………………………….… 14

SLC vs MLC ............................................................................................................................ 15

NAND Flash Memory Operation ............................................................................................. 15

How Hard Disks Work ............................................................................................................. 16

SSD Technical Breakthroughs ................................................................................................. 18

Performance Analysis ................................................................................................................19

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The Anatomy of Modern SSD ............................................................................................. 20

Disk Fragmentation ............................................................................................................... 25

When will SSD totally replace HDD ……………………………………………………....26

Conclusion .................................................................................................................................. 27

References .................................................................................................................................... 28

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Abstract

In recent years there has been a tremendous turn around in the capacity and cost of the storage

systems used in our computers. This tremendous growth resulted in an increase in the demands

for storage drives which then led to the introduction and adoption of a more advanced storage

system such as the solid state system (SSS).

The two main currently used storage systems are; solid state drives (SSDs) and hard disk drive

(HDDs). These storage systems have their various importance, similarities and dissimilarities,

advantages and disadvantages in terms of the used technology, latency, data reliability, power

consumption, data speed, storage capacity, cost and so on. SSS make use of a non-volatile

memory known as NAND Flash memory. It is called non-volatile because of the ability of the

storage drive to retain its data in case of power loss. The life span of a solid state drive is

predictable due to its finite erase cycle which causes the disk drive to wear out. NAND based

storage system was designed to provide solutions to the high cost of SSDs in the market, for

better data integrity, data reliability, to improve write/erase performance, increase product

capacities as well as to further lower the storage drive power consumption. etc. Some of these

requirements have been met as manufacturers are still finding it difficult to design SSSs that can

meet all the above requirements. Hence, different technology with better write performance and

lower cost than the NAND based technology is being considered for implementation if by chance

SSS fails as a viable technology for mass storage. Detailed discussion on this aspect and lot more

will be found in this paper as we go further. The technical breakthroughs involved in SSS will be

examined while this paper will conclude by looking at the possibilities of SSSs replacing HDDs

in our modern computer systems.

I. Introduction

The main technology in SSDs is NAND Flash. NAND flash memory is a form of Electronically-

Erasable Programmable Read-Only Memory (EEPROM). This technology share some features

with ROM and RAM. NAND flash is non-volatile like ROM. Which means data is retained in

the case of sudden power off. Also, data stored on NAND flash can be either deleted or erased.

Deleted data usually does not have much impact on the drive, as the space will not be seen as

free space by the controller even though the operating system (OS) understands what is going on.

It is therefore better to erase the data for new data to be written than to delete. This is similar to

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the mode of operation in RAM. 1). In NAND memory, overwritten is not supported. For new

data to be written to a page, stale data have to be erased. This is because Flash has a limited write

time 2). Flash has a limited erase cycle. Flash has erase limit of 3k to 10k times, with SLC

limited erase time being 100,000 (higher than MLC and TLC). This is according to the

manufacturing techniques and Flash types. 3). Flash’s smallest erase unit is block, while the

smallest write unit is page. It means that pages of data can be written to a flash but blocks of data

will have to be erased before further writing could be done in the flash area. This is done because

Flash does not support overwritten unlike HDD. Based on the characteristics listed above,

garbage collection technology needs to be adopted in order to gather the invalid data in Flash to

create some free space for further write operations to be carried out. NAND Flash based solid

state storage is now a key element of next generation data storage due to its outstanding

performances.

On the other hand, a lot of consumers don’t believe SSDs will ever replace HDDs. This

researcher as well does not believe SSDs will totally replace HDDs anytime soon due to its high

cost. Although, SSDs have faster data transfer speed because it has no moving parts as in HDDs.

This high product cost is the reason SSDs are not as popular as they should be today. However, if

some technical aspects of SSDs could be restructured such that will lead to an increase in the

write time, further increase the storage capacity, and further reduction in the product (SSD) cost,

there is a great probability that SSDs will occupy the storage market as HDDs is today and even

more. Consumers must note that SSDs cost will definitely not be as low as HDDs anytime soon

as the technology used in both storage drives are not same. Detailed explanations on SSS and

HDD will be found in this paper as the researcher proceeds further with the report.

THE HISTORY

Hard Disk Drives

The history of the hard disks (HDD) is dated back to the 1950’s. IBM produced the first

commercial usage of hard disk drives in 1956 called IBM 305 RAMAC with the storage capacity

of 5MB having fifty “24” platters. Those hard disks were larger in physical size rather capacity;

hence they needed a very big space to operate effectively.

Another development took place in 1973 when IBM introduced the IBM 3340 with the total

capacity 70MB. In 1980 IBM developed the first 1 GB hard disk and was named IBM 3380. In

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1986 the SCSI technology was standardized and was followed by the creation of the first PC disk

drives. From that time till date, the hard disk capacity has been increasing dramatically with

noticeable drop in price per GB. In 2004 a 250GB hard disk was introduced. In 2005, 500GB

hard drive was produced. In 2007 Hitachi released the first 1TB drive and in 2009 the capacity

has reached 2TB.consumers are benefitting a lot from this because more data are generated on

daily basis and a higher capacity storage drive is all they could ask for. However, some scientist

thinks that this technology has reached its limits and an end will soon be put to this tremendous

development. The first developed HDD is shown in the image below;

Image 1

Source: www.brightsideofnews.com/print/2011/12/7/your-storage-blog-make-storage-cheaper-

and-more-energy-efficient!.aspx

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Solid State Disk

SSDs have shorter history compared to HDD because it has not been too long it came along. The

extreme high cost of SSDs drew back its usage. As at 1970-1980, SSDs could not be used in

personal computers or even servers as it is used today due to it high cost. It was scientifically

used then in supercomputers by researchers. in the late 1900’s, another set of SSDs were

developed by StorageTek, but this time it was used for enterprise purposes. That is for mass

storage because individual couldn’t afford it yet. M-Systems introduced the first solid disk drives

in 1995 and in 1996 another storage company called ATTO introduced a solid state drive with a

capacity of 1.6GB in a 5.25” form. In 2001 SSD highest capacity reached 14 GB for flash 3.5.

The product cost for this was about $42,000. Despite the low capacity the product cost was

extremely high and individuals were still not able to afford it. In 2006 - the first laptop that used

SSD was launched by Samsung. That same year, USB sticks capacity had reached 4GB. Since

2007 many laptop manufacturers have been selling their models with an SSD option

Image 2

Source: www.storagereview.com/kingston_hyperx_3k_ssd_review

All About Solid State Storage (SSS)

Solid State Storage (SSS): this storage system is mostly found in computer systems. SSS are

produced from silicon microchips. These microchips are integrated together to form the solid

state storage system. In Solid State Storage, data are stored electronically rather than

magnetically as in the traditional Hard Disk Drives (HDD) with moving parts. SSS are not only

used in computer systems. They are also used in modern network server, which means they are

used for enterprise purposes as well. Although solid state storage itself is not new in technology,

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there has been recent interest in how the enterprise storage can be improved by this technology.

The increase in SSD processor speed which has been proven to have outweighed the non-

development in the traditional hard disks (HDDs) brought about this sudden interest. A very

important feature to note in Solid state storage (SSS) is the non-existence of moving parts or

spinning. SSS which is said to be an improvement on HDDs has no moving parts thereby data is

stored electronically rather than magnetically. Hence, SSS data transfer from the main memory

to the disk and from the disk to the main memory is done at a very high speed due to the zero

seek time in SSS.

SSS Form Factor

Solid State storage can be classified into three form factors:

a. Solid state drives (SSD)

b. SOLID STATE CARDS (SSC)

c. Solid state Modules (SSM)

A. SSD form factor is the physical configuration of Solid State Storage (SSS) media. This refers

to the interfaces used by SSDs such as SATA, SCSI, IDE etc. In past times, SSDs have been

observed to be more expensive than the traditional hard drives, but in recent years, there has been

a significant fall in the prices of SSD because of the outstanding improvements in its technology

and capacity. Although there have been some improvements in SSDs capacities of recent, one of

the main challenges the solid state drive is faced with is performance. Unlike the traditional hard

disk with unlimited read-write times, SSDs have finite write/erase cycle. Hence, overwritten is

not allowed. When the memory gets full or if more free space is required, the system searches for

the oldest (stale) file in the drive whose content is probably no longer useful and then erase the

file thereby creating more free space for data to be written to the memory page, while in HDDs,

data could be written several time on the disk because of the unlimited write time. The more time

data is erased from the disk, the shorter the life span of the disk. The product cost of HDD is

lower than SSDs even though its capacity is higher. As at 2013, HDD with the maximum storage

capacity of 2Terrabyte were manufactured for PC, while servers have HDD with storage

capacities up to 10TerraByte. SSD on the other hand is still trying to meet up. The maximum

storage space produced for SSD in 2013 was of 512GigaBytes. Even with this lesser disk space,

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SSDs are still lot more expensive than HDDs. SSDs read time is also faster than the read time in

HDDs.

B. Solid State Cards (SSC) uses the standard card form factors which can be found on a printed

circuit board such as Peripheral Interconnect express (PCIe).

C. Solid State Module is used on mobile phone where performance is of less importance rather

than size. Take for example the external memory used in digital camera or cell phones. That is

the micro scan disk. This is a form of SSM used nowadays. Solid state module resides in

memory modules such as Dual In-Line Memory Module (DIMM) .

Image 3

Source: www.blog.lifespantechnology.com/it-asset-disposition-blog/bid/302377/The-Successful-

Eradication-of-Data-on-Solid-State-Disks-SSDs

The two main components of the SSDs are the memory (storage drives) and the controller. In

previous years, the storage drive used in SSDs was DRAM volatile memory until 2009 when

NAND Flash non-volatile memory took over.

A. MEMORY (storage drives):

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The storage drive used in SSDs today is referred to as NAND Flash memory. NAND Flash is a

non-volatile memory which allows data on the disk to be retained regardless of if the device

power goes off or not. The previously used DRAM has the functions of a RAM whereby loss of

power results in loss of data. But NAND flash memory operates like a random access memory

(ROM). In a ROM the data on the disk are still intact even if the device power goes off.

B.CONTROLLER:

The controller is one the two main components of a Solid State Drive. It is an SSD integrated

processor that executes some command code at the firmware level. The controller performs

operations like:

i. Garbage Collection: in SSD data is written to flash memories in small units called pages (using

MLC), while the data is erased in a larger unit known as blocks. The write and erase operation

use multiple cells and multiple pages respectively. If old or stale pages that are no longer useful

are noticed in the blocks of data, the pages with useful data in the data blocks are read and

rewritten into empty blocks that were previously erased. The free pages available by moving the

stale pages of data will then be available for new data. This process is referred to as garbage

collection. Garbage collection is done to create more free space for new data to be read and

written by erasing stale pages.

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Image 4

Source: www.en.wikipedia.org/wiki/Write_amplification

ii.Wear levelling: this process is one of the operations of flash controller. As earlier explained

NAND flash has finite write cycle which makes the drive to wear out when it reaches its limit.

For example SLC NAND flash has 100,000 write cycles, once this limit is reached, the drive

becomes unreliable and data can no longer be written to the drive. In order to ensure evenly

distributed write times and erase cycle in all NAND memory sectors or data block, wear

levelling process is adopted. Wear levelling is designed to elongate the life span of SSD devices.

The controller makes use of wear levelling algorithm to select the block of data to be used

whenever data is read. Dynamic and static wear levelling are the two types of SSDs wear

levelling. In Static wear levelling the target block with the overall lowest erase count is selected

for the next write operation, while in dynamic wear levelling, previously erased blocks are

searched for, and the block with the least erase count is selected for the next write operation.

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iii. Address Mapping Technology

Address mapping technology is the method of mapping logical address to physical address. The

operating system generally write data according to HDD’s sector size (512bits) but Flash

smallest read/write unit is page (4KB), while the smallest erase unit is block (typically 128 or

256 pages). To proffer solution to this problem, Flash translation layer (FTL) between the

Operating system and storage medium is increased. There are three major types of mapping

technologies: 1) paged-based address mapping 2) block-based address mapping 3) address

mapping mixed. Page is the smallest unit in page based address mapping table. Block-based

address mapping table has block as its smallest unit, while in mixed address mapping, automatic

selection is done. Mixed address mapping selects block or page as its smallest unit, in

accordance to data updates frequency. Page based address mapping table has good performance,

high flexibility, low power consumption and high product cost, while the block-based mapping

table has low cost with low storage medium utilization. In a Flash area, whenever the file system

want to write data, the data is converted to physical address by FTL & write to free physical

page, the mapping table is updated while the original data is marked as invalid data. SSD

performance is therefore affected by the rate of conversion of FTL. FTL is used to translate LBA

address to FLASH address. To complete a valid writing as a result of re-written problem,

changes are made to the storage mapping table as many times as possible as well as data in the

physical storage unit. This increases the number of operations of FLASH which in return leads to

reduction in FLASH life expectancy as a result of limited erase times in FLASH.

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Image 5

Source: http://storagemojo.com/2008/07/15/design_tradeoffs_for_SSD_performance%20ii.

Types of SS SYSTEM

The two types of Solid State Systems used today are;

1. Flash memory based SSD

2. RAM – based SSD

Flash based SSD was introduced in 1995 by M-Systems.

The two types of flash memory are in used today. They are;

1. NAND flash memory

2. NOR flash memory

The higher capacity and faster read time of SSD is the main reason NAND Flash is employed in

enterprise today. Before the use of NAND flash memories in SSDs, DRAM volatile flash

memories were in use. DRAM stores data electrically and requires constant electricity to retain

its data in case of sudden loss of power. Most network servers nowadays make use of NAND

flash because of the above stated features thereby giving little priority to the product cost. NAND

flash is a non-volatile flash memory which has the capacity to retain data stored in the case of

power loss. On the other hand, RAM-based solid state system is volatile. The operation in RAM-

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based systems could be likened to that of HDD. In RAM-based Storage systems, data write cycle

is unlimited. Which means the storage system doesn’t wear out.

In Flash-based systems, data write cycle is finite; therefore it is possible to predict the lifespan of

the media before it wears out. Overwritten is not allowed in flash memories.

Types of NAND Flash

There are also three types of NAND flash;

1. Single level cell (SLC )

2. Multi-level cell (MLC) and

3. Triple level cell (TLC)

A.SLC or MLC is used in Flash-based systems. Data are stored in single level cell or muti-level

cell in Flash-based systems. SLC NAND stores two states per memory cell and allows a single

bit of data to be written per memory cell. The states are one (1) or zero (0). Although, SLC based

systems have higher speed and more robust than TLC and MLC, it has lower storage capacity

and more expensive than TLC and MLC. This makes it not widely accepted by manufacturers.

B. MLC NAND allows four states of data to be stored per memory cell and two data segments to

be written per memory cell. MLC States are 10, 11, 01 and 00. Contrary to SLC, it has higher

storage capacity even though it has slower speed and less robust than SLC. MLC NAND is used

today in SSDs because of its higher storage capacity and lower cost. Its storage capacity is twice

that of SLC.

C. TLC NAND is not yet in operation. It is designed for future technology. TLC NAND stores 8

states per memory cell. That is, 101, 110, 111, 000, 001, 010, 011, and 100. It allows 3bits of

data to be written by memory cell. TLC possesses even higher storage capacity than SLC and

MLC. It storage density is thrice that of SLC. The multi-layer flash is the slowest of the three

types of NAND flash. Researchers predicted that by 2015, MLC NAND flash will later be used

in SSDs. This is probably as a result of its high storage capacity. However, TLC-based SSDs

may not be able to overtake MLC and may not be considered as a general storage drive due to its

performance and limited lifespan even though the product cost is low. Just as manufacturers are

working hard to ensure hybrid storage system exist. That is, a combination of HDD and SDD

technology on a single drive, manufacturers are also working on hybrid SSD, which is a

combination of SLC and MLC technology in a single Flash memory chip for better memory chip

performance.

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.

Table 1

Source: www.arstechnica.com/gadgets/2008/11/sandisk-plans-hat-trick-of-SSD-performance-

improvements/

MODE of OPERATION in NAND FLASH MEMORY

An SSD has a small memory called cache where activity such as increase in the performance of

SSD is done by temporarily storing frequently accessed data. Cache is an important component

of SSD as stated by ”Rick Cook”, a storage expert; One of the most potent use of solid state

devices (SSDs) is employing the drives as a super cache to speed up access to frequently

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accessed files or applications. In enterprise SSS, NAND flash technology is used because of its

higher capacity as well as its faster write/erase times.

To prevent data loss due to sudden power off, the cache data must periodically write back to the

NAND Flash memory. A flush command is issued immediately the cache data writes back to the

NAND Flash memory. The cache data immediately flush all dirty data item into a non-volatile

storage media.

A NAND Flash memory is a non-volatile storage media where all dirty data item from the cache

is flushed into to create enough blocks space for the next data to be written. This process is

referred to as garbage collection. Garbage collection, a major technology for SSD has a lot of

influence on SSDs’ mode of operation. Garbage Collection (GC) algorithm has high cost and

low efficiency problem.

SSD interfaces like Serial Advanced Technology Attached (SATA) and Serial Attached Small

Computer System Input (SAS) support flush command. Unlike Hard disk drive (HDD), SSD has

the ability to retain its data in the event of power loss, acting like a Read Only Memory (ROM).

Hard Disk Drives (HDD)

How Hard Drives Work

HDD is a non-volatile memory device that stores data magnetically. HDD has a rotor which

holds a small number of flat circular disks. These disks are called platters and data is recorded on

them. The platters spin at great speed often at 7200RPM for a desktop hard drive and 5400RPM

for laptop. A device called the head scans the surface of the spinning platters and reads, writes or

modifies existing data. The head is attached on an arm which moves it from one edge of the

platter to the other very fast, sometimes up to 50 times every second. The number of heads

depends on the number of platters inside the unit. Modern HDD platters store data on both sides

and that is why two heads are used for a single platter.

The surface of the platter is divided into concentric circles called tracks. There are a number of

sections per track forming sectors. A cluster is a group of sectors and in case of multiple platters

inside one unit; we talk about cylinders instead of tracks. Between the sectors there are some

unused areas called intersector gaps, similarly there are the intertrack gaps between the tracks.

The most important characteristic of the hard disk is the moving parts: the spinning platters and

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the moving heads. The speed of the rotating platters usually varies from 4500 to 7200 RPM.

Seek time is measured in milliseconds.

All modern hard disks contain a secondary memory called cache or buffer. This is a way to

increase the performance of the hard disk .This kind of memory works similarly to the other

caches of the personal computers but it is rarely included in the hierarchy of the PC memories.

The purpose of the cache is to keep the results of the most recent requests in case a similar

request arrives. Similarly, during a write operation, the data can pushed into the cache much

faster and the system bus can be released while the HDD internally completes the operation. This

technique reduce the number of physical accesses to the disk and improves overall performance

.The most recent hard disks use 16 MB of internal cache memory.

Image 6

Source: www.examiner.com/article/smr-hamr-tech-for-future-HDDs

1. Seek time: this is one of the three key areas in reading and writing data to a disk drive (HDD).

The others are transfer time and rotational time. For data to be read and written to a particular

sector on the disk, the read/write head of the disk as shown in the pictures above must be moved

to the appropriate place. This process ids referred to as seeking while the time taken for the head

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to move to the correct place is the seek time. Seek time is the time taken for a hard disk

controller to locate specific piece of stored data.

2. Rotational delay: this is time taken for the addressed area of the disk to move to the correct

place where it will be easily accessible to read/write head.

3. Access time: this is the time taken for computer to process data sent from the processor and

then retrieves required data from a storage device.

SSD BREAKTHROUGHS

SSDs started gaining public recognition as soon as its mass production for PCs emerged. The

first SSD which was manufactured by IBM was designed for servers and until it was designed

for PC, SSD was not widely recognized.

Apart from SSD lower power consumption, SSDs are more reliable than HDD. However, the

lower or smaller read latency time of about 50MB/s – 100MB/s in SSD compared to the high

read latency time present in HDD is one of the factor that brought about the breakthroughs in

SSD. Even though HDD has a faster write time compared to SSD, that does not give it an edge

over SSD as SSD read time is faster. A lot of consumers are after faster read time which makes

SSD a better option. Over the years, a major challenge encountered in storage systems is the high

power consumption rate. However, the introduction of SSD has proffered solution to that

problem, hence increased the amount of activities that can be carried out within a time limit.

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Table 2:

Source: www.itecholic.com/SSD-vs-HDD-which-one-to-get/

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SSD and HDD Performance Analysis

This has to do with the performing testing of HDD. The following areas are to be considered;

1. Seek time: This is the most important aspect to be considered in hard disks. Seek time is the

time taken for a hard disk controller to locate specific piece of stored data.

2. Sequential Read/Write Performance: This is used to test the performance of the storage drive

when data are read or written to the storage drive sequentially. This is commonly used in SSDs

as data are stored randomly in HDDs.

3. Rotational Latency: since modern hard disks rotate at very high speed, there will be no

noticeable delay experienced.

4. Cost per GB: this shows the cost of the data capacity of the device. The product cost SSDs is

often higher than for HDDs.

5. Capacity: This indicates the overall amount of data that can be stored on the drive.

6. Random Read/Write Performance: this performance metric is used to when data is read or

written in a random manner to the disk drive. It is used to check HDD performances as data are

written randomly to the disk drive.

UNDERSTANDING HOW THE MODERN STORAGE SYSTEM (SSD) WORKS

The analysis of this mass storage device is important in order to have a better understanding of

the mode of operation in the storage device. We will be examining a typical solid state drive.

NAND MLC is widely used SSD memory type in our modern devices because of its higher

capacity which is what a lot of people long for. It appears to be one of the basic needs for

modern computer systems. Each MLC store one or two bits of data per memory cell. These cells

are organised into unit called page. Currently, the standard size of a page is 4KB. Pages are

further combined together to form blocks of 128 pages which results in 512KB (128x4) per

block. Technically, the same memory circuits used in common USB sticks are also used in SSD

devices. In order words, the average USB stick offers 25MB to 40MB per second in terms of

read speed and write speed can probably be lower than that. SSD read time is faster than this

because each SSD has multiple memory circuit used as opposed only one used in HDD.

The Performance problems in SSDs erupted from the fact that a block is the smallest structure

that an SSD can erase. The controller on the SSD can write/read to/from a single page but for

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data to be deleted, the entire block has to be erased before the whole block can be re-written. The

worse scenario is the fact that each time a block is erased the lifespan of the block is reduced.

Based on these challenges, two things must be avoided in SSDs: 1) avoid deleting data and 2)

avoid erasing data.

The proffer solution to this challenge, an algorithm must be implemented in the SSD controller

which will enable it function more like HDD. It will keep writing to new blocks every time the

write operation is requested instead of overwriting the old ones. By so doing, wear leveling will

take place and every block in the entire disk will at least be written to once before the disk starts

rewriting blocks and thus reducing their life span. Similarly, when deleting files, the block will

be marked as invalid but it will never be erased. This solves the problem with the life span of

SSD because most users never completely fill up their drives and the empty space can be used to

prolong the life of the other cells. So if we consider a 32GB SSD, to reduce the life of every cell

in the disk by one cycle, the user will actually have to write more than 32GB to the disk.

Unfortunately this solution introduces a performance problem.

The performance problem comes from the fact that the operating system does not understand

how the SSD controller performs the data erase operation. It is however completely transparent

to the operating system whether the SSD is writing to an empty block or it is overwriting a block.

Bearing in mind that if it takes the SSD X amount of time to write to an empty block ,it will take

X+Y to write to a block that has already been used and was marked as invalid, where Y will be

the time to erase that block.

Using the illustration below by Anand Lal Shimpi from Anandtech in his article “The SSD

Anthology” which was published on March 18th 2008, he used the concept of an hypothetical

SSD with characteristics shown in the table 3 below,

Our hypothetical SSD

Page size 4KB

Block size 5 Pages (20KB)

Drive size I Block (20 KB)

Read speed 2KB/s

Write speed 1KB/s

Table 3

Source: www.anandtech.com/show/2738/8

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We are assuming a solid state disk with a page size of 4KB and a block size of 5 pages. The total

drive size is 1 block and we can read from this disk at 2KB/s and write at 1KB/s. which means

we have faster read time than write time. When we try to save a 4KB file to the disk, the SSD

will store it in a single page as shown in image 7 below;

Image 7

Source: www.anandtech.com/show/2738/8

To the operating system (OS), this write operation will occur at 1KB/s since all our pages are

empty initially. Also, the next write operation of an 8KB file will also occur at 1KB/s and will be

written to next two free available pages.

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Image 8

Source: www.anandtech.com/show/2738/8

However, if the user decide to delete the document so the SSD controller marked the page as

invalid but not really erasing the block because that would lead to a reduction in the life cycle of

the disk. The user will like to write a new file of 12KB to the disk. The controller been aware

that the remaining free space is not enough to write the entire 12KB file to the disk, will then

have to erase the entire data block for more free space to be created and for new data to be

written. The controller will then have to transfer the old (existing) file to the memory cache and

then delete the entire block. It then re-writes the old file and writes the new file to the drive. By

so doing a data file of 20KB has been written to the page. This operation is transparent to the

operating system which initially thought it is only writing 12KB at a speed of 1KB/s. What

suddenly happened is that our SSD took longer to write the new data and this is perceived as

lower performance by the operating system and the user. This of course does not mean that a

solid state storage device that is faster than a hard disk will eventually be slower. It does mean

however, that once such a device has had its pages filled with data the performance will degrade

as it will need to erase the blocks to write new data.

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Image 9 below illustrates the entire operations;

Image 9

Source: www.anandtech.com/show/2738/8

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Hard disks also suffer from a similar problem but for different reasons. Hard disks start to write

data on the outer tracks of the platters. The velocity at the outer side of the platters is of course

higher than the spinning velocity closer to the centre and the head can cover a greater area for the

same degrees of rotation. This means that as the disk gets full and data has to be written closer to

the centre of the platter, the worse the transfer speeds will become.

There is a way to overcome this performance problem with SSDs but it comes at the cost of

reducing the life cycle count. The TRIM command can be used to forcefully delete all the invalid

cells in a solid state disk. The command is not yet supported by operating systems but there are

utilities available that wipe the data off SSDs

DISK FRAGMENTATION

Disk fragmentation occurs in both solid state memory and hard disk drive. Although, SSD is not

affected by disk fragmentation because it has no moving parts, hence, data retrieval in SSDs is

faster than in HDDs. HDDs suffer lot more from disk fragmentation. In HDD, files are created,

deleted and modified and new data are written. Most times a file is often divided into the free

spaces available on the disk. This is a natural process in disk drives (HDDs) that occurs when a

disk is frequently used. Fragmentation slows down data access speed because the entire disk

drive has to be search for a single file to be put together. To improve on the performance of the

disk drive, disk defragmentation has to be done. Defragmentation is the process of assembly the

scattered files into a single block. This process increases the disk drive speed as data are now

located in the same page. Fragmentation and defragmentation processes are as shown in the

picture below.

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Source: www.news.techgenie.com/latest/how-to-defragment-your-disk-drive-volumes-in-

windows-xp

WHEN DO YOU THINK SSD WILL TOTALLY REPLACE HDD?

First and foremost, I do not think SSD can totally replace HDD. The HDDs we use presently

have gone far ahead of the SSDs in terms of product cost and disk capacity. As at 2013, HDD

with the maximum storage capacity of 1terrabyte were manufactured for PC, while servers have

HDD with storage capacities up to 10TerraByte. SSD on the other hand is still trying to meet up.

The maximum storage space produced for SSD in 2013 was of 512GigaBytes.

However, some might not think this is a necessary criterion as SSD is more reliable and

has higher read latency than HDD. But, who really cares if SSD random read is better off

HDD’S. What majority care about this day is how large the storage space on the PC is, and this

has given rise to questions like: Can I run this application on my PC or not? Can I save myself

some money by downloading movies and music on my PC or not.

Research has it that manufacturers are working very hard to ensure both SSD technology

and HDD technology both exist on a single disk, which I think is a really good idea if they can

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get it to function well. But honestly if you ask me, I will say SSD cannot totally replace HDD in

20years to come.

CONCLUSION

A lot of consumers are still faced with the choice of which of these two powerful storage drives

should they consider while purchasing a disk drive. This actually does not have an exact answer

as two key areas must be considered. The product cost and the performance. As clearly explained

in this report, the cost of SSDs may be high but it performances outweighs that of HDDs. So for

a user that long for a faster performing drive without considering the high cost, SSD will be a

good choice. However, if the product cost is to be considered lot more than the performance,

then HDD will be a better choice. So it all depends on the consumer what his priority is.

However, researches have predicted that by 2024 which is a decade from now, SSDs speed will

decrease drastically as a result of increase in latency. The delay time is expected to increase by at

least 2.5 times over what it is presently. Although we expect continuous growth in the storage

system capacity, the decrease in performance will make SSDs unattractive to consumers. Hence,

it becomes a less viable technology for some applications. Nevertheless, IBM researchers have

discovered a new technology with more write times and better performance than SSDs. This

discovered technology is the phase-change memory (PCM) with 100million write/erase cycles.

Which is 10 times greater than the flash memory used today (3,000 to 10,000 write/erase cycle).

This may probably be implemented in the future if SSDs fails as a viable technology for mass

storage.

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REFERENCES

1.Choi, G.S. & On, B.-W., 2011. Study of the performance impact of a cache buffer in solid-state

disks. Microprocessors and Microsystems, 35(3), pp.359–369

2. http://www.extremetech.com/computing/118909-current-solid-state-drive-technology-is-

doomed-researchers-say

3. http://codecapsule.com/2014/02/12/coding-for-SSDs-part-2-architecture-of-an-SSD-and-

benchmarking/

4. http://codecapsule.com/2014/02/12/coding-for-SSDs-part-2-architecture-of-an-SSD-and-

benchmarking/

5. http://www.pcmag.com/article2/0,2817,2387923,00.asp

6. http://www.igcseict.info/theory/3/solid/

7. http://searchstorage.techtarget.co.uk/definition/Multi-level-cell-MLC-flash

8. http://searchsolidstatestorage.techtarget.com/definition/wear-leveling

9. www.en.wikipedia.org/wiki/Write_amplification

10. http://www.techopedia.com/definition/3558/seek-time

11. http://news.techgenie.com/latest/how-to-defragment-your-disk-drive-volumes-in-windows-

xp/

12. www.blog.lifespantechnology.com/it-asset-disposition-blog/bid/302377/The-Successful-

Eradication-of-Data-on-Solid-State-Disks-SSDs

13. http://www.brightsideofnews.com/print/2011/12/7/your-storage-blog-make-storage-cheaper-

and-more-energy-efficient!.aspx