COMP3013
C
ONFERENCE
C
OMPUTING
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HDD
VS
SSD
IS THE
F
UTURE OF
HDD
A
LMOST
O
VER
?
George Chrysotis
Electronics and Computer ScienceUniversity of Southampton Southampton,UK [email protected] users.ecs.soton.ac.uk/gc5v07
A
BSTRACT
This paper analyses in depth the structure of the conventional Hard Disk Drives (HDDs) and the more modern Solid State drives (SSDs). The paper presents different materials and architectures applied in HDDs over the years, aiming to increase data capacity and improve responsiveness and durability. By comparing the physiology and the technical characteristics, the paper aims to identify the one which will dominate the market in the near future.
Keywords
HDDs, SSDs, future, materials, architectures, flash.
1. I
NTRODUCTION
This paper presents in section 2, a very brief background on both Hard Disk Drives and Solid State Drives. Then in section 3 the paper presents HDD architecture, followed by the physical characteristics of HDDs (section 3.1), improvement techniques (section 3.2), recording techniques (sections 3.3 and 3.4) and possible future developments (3.5). Later on in section 4 Solid State Drives are presented, followed by an analysis on how FLASH stores information (4.1), a description of the NAND architecture implemented on SSDs (4.2), a presentation of the benefits of SSDs over HDDs (4.3), and the overheads of SSDs (4.4).
2. B
ACKGROUND
In 1953 the first HDD was developed by IBM known as the “IBM 305 RAMAC” consisting of 50 aluminium disks painted with iron oxide, each 2 feet
in diameter and weighting around a ton. The total enormous storage capacity of the drive could reach up to 5 million characters (today’s terms 4.4 MB). Very soon, in mid 1950’s the potential of HDD for development was observed and it already was fighting against its stronger competitor, the silicon based Dynamic Random Access Memory (DRAM). DRAM was developed as non-volatile using Electrically Erasable Programmable Read Only Memory (EEPROM) technology. As DRAM’s size increased over the years at a greater rate, it was estimated that DRAM would dominate over HDDs by mid 1990’s. However that was not the case. New architectures and changes in the elements composing the hard disk drives allowed HDDs to, not only catch up to the DRAM’s development, but increase their growth rate to significantly greater than that of the DRAM. [1]
3. HDD
S
Hard Disk Drives consist of three main parts. The platter, the actuator arm and the head as shown in Figure 1.
Figure 1 Basic HDD Diagram
Source: How to Recover Data From a Compaq Presario Hard Drive, By Jason Gordon, eHow
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3.1 Hard Disk Drive Physiology
The platter is a disk consists from two layers, the substrate material and the media layer. The substrate material is usually constructed from aluminium, and is used as Charles M. Kozieroc suggest in his analysis on Hard Disk Drives [2], to form a “rigid, lightweight, stable, magnetically inert and inexpensive disk”. IBM on March 15th 2000 announced to have successfully developed the first desktop drive to use glass disk platters instead of aluminium1. Other materials for platter construction include magnesium alloys, glass and ceramic composites, and titanium. Each of these materials has different advantages in terms of heat distribution and more importantly surface structure i.e. smoothness. As shown in Figure 2 there is a huge difference between an aluminium platter and a glass one when viewed under an electron microscope. As disk heads tried to fly closer and closer to the rotating platter smoother surfaces were an essential need to avoid head crashes. Also materials such as titanium allow thinner platters with the same durability. This can eventually create room for more platters, more surface area and therefore more data storage area. As shown in Figure 3 platters can be stacked and with the use of multiple heads, multiple simultaneous reads and writes can be achieved.
Figure 2 Surface of aluminum platter (left) vs. surface of a glass plater (right)
Source: Plater Substrate materials, by Charles M. Kozierok © IBM Corporation
http://www.pcguide.com/ref/hdd/op/z_ibm_microscop e.jpg
The media layer consists of iron oxide – essentially rust which is applied on the disk surface. Iron oxide proved to be quite vulnerable in two areas. The first one is that on head crashes oxide particles could easily get detached from the surface of the disk meaning loss of data. The second is that the data density an iron oxide disk could store was relatively low. Iron oxide was replaced by an even thinner and
1
IBM Announces World’s Highest Capacity Hard Drive, Press Releases, San Jose, Calif, March 15th 2000
http://www-03.ibm.com/press/us/en/pressrelease/1826.wss
smoother magnetic material layer known as “thin film media” consisting of a very thin magnetic material which was more durable on head crashes and allowed increased data density. The media layer is separated into sectors and tracks as shown in Figure 3. Data position can be identified on a platter by track number and sector number.
Figure 3 Tracks Sectors and Clusters Source: Disk Partition Alignment Best Practices for SQL Server (Appendix: Disk Partitioning Alignment Internals), By Jimmy May and Denny Lee, May 2009.
The actuator arm or more commonly known as the access arm is a mechanical arm that moves the read/write head across the platter attempting to correctly place the head over a specific sector on the platter.
The head is the most expensive and sophisticated piece on a HDD. Although the concept of transforming electric pulses to magnetic signals is relatively straight forward, their manufacturing size and accuracy is what makes them so valuable. Initially the head was constructed by passing current through a coil and producing a magnetic field (write). By applying then that magnetic field to the coil caused electrical current to flow (read).
Section 3.2 describes the first architectural change that contributed towards the survival of HDDs. Sections 3.3, 3.4 and 3.5 present later and also future architectures planed to be applied on HDDs in order to increase density of storage.[2]
3.2 Flying the head closer to the platter
At the very early stages of HDD the heads were in contact with the magnetic material, however as this caused increased wearing, especially from track changes, and also great heat production, the design changed to a floating one. IBM in 1962 produced the 1301 which was the first model with a floating head. The flying height was at 6.32 micrometers. The first observation made was that by getting the head closer to the platter surface, the density of data
could increase significantly allowing more data to be written on a given area. This is where the need for smoother surfaces arose (discussed in section 3) as flying heights have now reached approximately 0.3 micrometers, as is claimed by Samsung’s white papers2.
Charles M. Kozieroc suggest that “As the flying height of drives continues to decrease, hard disk engineers are recognizing that we may soon reach the point where it cannot be made any smaller without touching the surfaces of the platters. There is actually talk about the possibility of going back to the concept of contact disks, where the head gap is intentionally made zero. This would allow even smaller magnetic fields than is possible in today's drives.”
3.3 Longitudal Recording
Initially HDDs heads as mentioned towards the end of section 3 were made from a coil. In order to write data a current was send through the coil in one direction producing a magnetic field, attracting the electrons found on the recording layer. Similarly to erase, (essentially a different kind of write) opposite direction current, was sent though the coil in order to create an opposite type of magnetic field, which repel electrons on the recording layer. In that way the sectors were identified and a binary code could be written on the recording layer. A read could then be accomplished by hovering the coil over the recording layer and checking the direction of the current produced by the magnetic fields stored on the recording layer. See Figure 4.
Figure 4 Longitudal Recording Source: Perpendicular recording
http://en.wikipedia.org/wiki/Perpendicular_magnetizat ion
3.4 Perpendicular Recording
Soon enough the areal density, defined as the amount of information that can be packed into an area of storage media3, reached its limits and there
2
SSB&IMPACGUARD
http://www.samsung.com/global/business/hdd/learningres ource/whitepapers/LearningResource_SSBImpacGuard .html
3
Smart Computing Dictionary
http://www.smartcomputing.com/editorial/dictionary/detail. asp
was a need for a new way to record on the magnetic layers. In mid 1980’s IBM using Magnetoresistive effect (MR) and the Giant Magnetorisistive effect (GMR) invented by the nobelist Sir Nevill Mott, created a new kind of heads that allowed areal density to grow. Data density reached two orders of magnitude greater than what was in 1980’s. The application of Giant Magnetoresistive on hard disk
drives was named as Perpendicular
Magnetoresistive (PMR). PMR has the advantage that it can deliver more than 3 times data density of traditional longitudinal recording. PMR also known as tunnelling Magnetoresistive (TMR).
The main difference with longitudinal recording is as observed in Figure 5, that perpendicular recording penetrates deeper in the recording layer at a perpendicular direction. This is achieved by the Magnetoresistive quantum mechanical effect and therefore less area is needed for one sector [3].
Figure 5 Perpendicular Recording Source: Perpendicular recording
http://en.wikipedia.org/wiki/Perpendicular_magnetizat ion
3.5 Future Recording
The last generation to be developed on the concept of PMR is claimed to be the Current Perpendicular to Plane Giant Magnetoresistance (CPP-GMR) which will deliver densities of 500Gbit/sq inch to 1Tbit/sq inch. Discrete Track Recording DTR and Bit Patterned Media BPM will be areas which will inevitably need to be developed. DTR will be achieved by nano-imprint technology and BPM “involves creating nano-scale pillars of magnetic material around 12-20nm high on the platter’s surface”. As the furthest prediction on technological evolution in HDD data recording is the Heat- Assisted Magnetoresistive Recording (HAMR) which is researched for years know. [4]
HAMP technology not only uses Magnetoresistive Recording on a recording layer, it also uses laser thermal assistance to first heat the material. This technique increases the areal density as “by heating a spot, a bit can be recorded more easily and the subsequent cooling will stabilize the data”4.
Another possible evolution of hard disk drives as presented by Sriram Sankar et al [5] last year
4
Humphrey Cheung, New Fujitsu optical head promises terabit per sq inch recording. http://www.tgdaily.com/ hardware-features/30024-new-fujitsu-optical-head-promises-terabit-per-square-inch-recording
claiming that “its time has come” is Intra-Disk parallelism. Essentially what they suggest is using multiple heads (up to 4) on platters to minimize seek time. They show in their research that it is possible by using 2 arm assemblies in a diagonal way to achieve simultaneous reads/writes from a single platter’s surface. Extending their idea to using two heads per arm they claim by their simulations, that it is also possible to have 4 simultaneous read/writes per platter’s surface.
4. SSD
S
Solid Stated Drives are essentially around since HDDs were invented. At their earliest state solid SSDs were known as Electrically Erasable Programmable Read Only Memory (EEPROM). Although the ROM part suggested that data can only be read from them, the fact that they are “electrically erasable” makes them applicable for a data storage device, as both read and write functions, are essential in a storage device.
SSDs have been designed with different interfaces that emulate a hard disk drive and this, makes their installation trivial. SSD interfaces as presented by Don Barnetson, Senior director of Marketing in SanDisk in his talk on “SSD and HDD Terminology and Architecture” [7] include Parallel ATA (PATA) operating up to 133MB/s, Serial ATA (SATA) versions 1-3 operating at 125, 250 and 500MB/s respectively and PCI Express (PCIe) v2.0 operating at 500MB/s per lane with very low power consumption.
4.1 FLASH
FLASH storage devices are metal oxide
semiconductor field effect transistors (MOSFETS) with floating gates also known as FGMOSFETS. The way information is stored on FLASH drives in order to make them non-volatile is far different from HDDs which use magnetic fields and current direction. A floating gate is achieved by connecting an n-type MOSFET with two gates on the source and the drain. The two gates are separated by a thin layer of oxide. When wanting to program a floating gate, i.e. store data, a large voltage is applied
between the source and the drain, and
simultaneously a larger voltage is applied to the control gate. In a NOR FLASH system this voltage generates electrons with high energy and these are located in the floating gate. In a NAND system though, this voltage causes quantum mechanical tunnelling to occur. The presence of metal oxide acts to repel further electrons from the substrate. In this way the threshold voltage of a programmed transistor will be higher (logic 0) than the non-programmed one (logic 1). Logic 1 is naturally while uncharged. [6]
Figure 6 Floating Gate Transistor Source:
http://en.wikipedia.org/wiki/Floating-gate_transistor
To read a cell a voltage is applied. In one case we get conduction in the other we do not.
4.2 NAND Architecture
NAND based architecture is preferred from NOR in the application of FLASH storage device on SSDs, as it allows denser storage. A NAND based SSD consists of several NAND FLASH storage modules connected on a bus (Figure 7 right) with the main controller. NAND FLASH is where the actual data are stored, how exactly they are stored has been analysed in the previous section, and the only thing left to mention is that we can have single level cell (SLC) or multi level cell (MLC). In SLC one bit is stored per cell, in MLC two or more bits can be stored on a single cell.
The controller on the left consists of the Error detection and Correction Checksum (ECC) which identifies and corrects data, a microcontroller and a number of Flash interface modules (FIMs). The microcontroller manipulates and moves data from the ECC to the DRAM and the FIMs. The DRAM is used as a buffer to enhance the performance of the microcontroller and FIMs are the interface between NAND modules and the microcontroller. The more FIMs the greater the parallelism, and therefore the greater the overall performance of the system. The Controller distributes the data from the SATA interface to the NAND modules and from the NAND modules to the SATA interface making the FLASH drive look more like a HDD.
Figure 7 NAND based SSD architecture Source: [7]
4.3 SDD benefits over HDD
SSDs are simply electronic chips mounted on a silicon board. They do not involve any mechanical parts moving around so they are more durable than an HDD. As they are purely electronic in contrast to electromechanical, they do not have waiting latencies involved while reading and writing and this is what makes them an order of magnitude faster than HDDS [8]. Also they are shock resistant and have a significantly lighter weight than HDDs. Energy consumption of SSD can be kept very low in comparison to HDDs, as Hung-Wei Tseng et al suggest in their research. An average energy saving of 20% can be achieved by the “subpaging technique”, 24,7% by “multiprogramming” and the “join of TFL storage buffer and subpaging” can reduce up to 35.6% flash memory energy consumption [9]. Other researches, such as switching on-demand on-off large magnetic disks, have been attempted in order to lower energy consumption, but they could not achieve such high results [12].
To study the benefits of SSDs in more depth we need to consider their application on real scenarios. Several researches have taken place in order to measure the differences between HDDs and SSDs. [8], [11] ]. Sang-Won Lee et al in their research on the application of SSD in Enterprise Database Applications, have “observed more than a magnitude improvement in transaction throughput and response time, by replacing a magnetic disk with a flash memory SSD for transaction log and rollback tables. More than a factor of two improvement in response time was also observed, for processing sort-merge or hash join query for temporary tables.” [8] Similarly, Professor Dae-Sik Ko and Seung-Kook Cheong, in their study on Enhancing E-Business using SSDs [11] last year, observed that by choosing the best tuning, SQL queries can be made as efficient as possible in their use of external storage, and by incorporating RAM based storage, latency is reduced by a factor of 100. Brian Beard, marketing manager for Samsung's flash memory group, in his interview by Lukas Mearian, from Computer World, suggests another very interesting scenario. As SSDs are in essence multiple NAND Chips, "moving forward people will design the entire notebook around SSD. You could spread SSD out over the mother board. So moving forward there will be a lot of custom notebooks with custom SSDs"5.
5
Laptops, desktops won't see a cost/benefit advantage in SSD for about two years, by Lucas Mearian, August 27, 2008. Published by ComputerWorld
http://www.computerworld.com/s/article/9112065/S olid_state_disk_lackluster_for_laptops_PCs
4.4 Overheads of SSDs
Apart from their high cost at the moment which is still almost double price per MB in comparison to HDD, NAND FLASH has also a major disadvantage. It is wearing out with use and therefore it has a life limit. Single layer cell (SLC) based FLASH devices have a limit of 100K writes, while Multi Layer Cell (MLC) based FLASH have a limit of10K writes [10]. Techniques such as the history based indexing have been developed in order to spread out the weathering across all cells that extend the life limit of SSDs to a maximum but still these techniques do not eliminate the wearing out.
5. C
ONCLUSION
By observing the characteristics of the two technologies in regards to the future of the market, one can easily at first conclude that SSDs will dominate over HDDs in the near future. They provide a significantly better random access memory performance, have a much better liability and a greater durability over Hard Disk Drives. However, the depth and analysis of this paper shows that big amounts of research is still undergoing for Hard Disk Drive technologies, such as the Heat Assisted Magnetoresistive Recording and therefore the near future development of HDDs is almost certain. If research provides economic solutions to what is required, ie denser data per platter and faster access times, possibly with multiple head technologies, then HDDs will be kept around for the near future as they do not wear out with use.
The final conclusion therefore is that there is a big potential for hard disk drives to be kept in the market for the near future but eventually in the further future they will be replaced by SSDs. The main reason is that electronic devices tend to become smaller and digital, therefore no room for rotating platters is available. As Lukas Mearian suggests, by changing the architecture of a computer a “distributed SSD” would be an ideal development for the future.
6. R
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