GIT – SSD (Solid State Drive – Ổ lưu trữ bán dẫn) là một thiết bị lưu trữ sử dụng bộ nhớ flash để lưu trữ dữ liệu trên máy tính một cách bền vững. Một ổ SSD đồng thời mô phỏng quá trình lưu trữ và truy cập dữ liệu giống như ổ đĩa cứng (HDD) thông thường và do đó dễ dàng được sử dụng cho nhiều mục đích khác nhau. Ổ SSD sử dụng SRAM hoặc DRAM hoặc bộ nhớ FLash để lưu dữ liệu, không nên nhầm lẫn với RAM Disk là một công nghệ mô phỏng và lưu dữ liệu trên RAM.
Nhờ việc sử dụng RAM để lưu dữ liệu, hoạt động đọc/ghi dữ liệu của SSD không kéo theo sự chuyển động của bất cứ phần nào trên ổ đĩa và do đó làm ổ đĩa bền vững hơn so với HDD, gần như không gây tiếng ồn, không có độ trễ cơ học nên mang lại tốc độ truy cập cao hơn. Đồng thời không mất thời gian khởi động như ổ HDD.
Ngoài ra, nhờ không sử dụng đầu đọc cơ học để truy cập dữ liệu, SSD tiêu tốn ít điện năng hơn HDD và có thể hoạt động ở điện áp thấp hơn so với HDD, kích thước gọn hơn. Do đó, nó được sử dụng trong nhiều máy tính điện áp thấp. Ổ SSD của Texas Instrument sử dụng RAM có thời gian truy cập dữ liệu là 15 micro giây, nhanh gấp 250 lần ổ cứng truyền thống, còn ổ SSD sử dụng bộ nhớ flash có thời gian truy cập dữ liệu từ 80-120 micro giây.
Ổ SSD có dải hoạt động nhiệt cao hơn HDD, thông thường trong dải nhiệt 5-55oC. Một số ổ flash có thể hoạt động ở nhiệt độ 70oC. Tuy nhiên, ổ SSD có những hạn chế về dung lượng lưu trữ, độ bền đọc/ghi so với ổ HDD thông thường. Hiện nay một ổ SSD dạng Flash có thể đọc ghi tối đa khoảng 10.000 lần cho ổ loại MLC và 100.000 lần cho ổ loại SLC. Ổ SSD đắt hơn nhiều lần so với HDD nếu tính trên đơn vị dung lượng lưu trữ.
Thiết bị SSD đầu tiên sử dụng sắt từ được tạo ra vào thời kỳ máy tính còn sử dụng ống chân không. Tuy nhiên với sự xuất hiện của ở lưu trữ dạng trống, nó bị ngừng sử dụng. Sau đó, vào những năm 70-80 của thế kỷ 20, ổ SSD tiếp tục được phát triển bởi IBM, Amdahl và Cray. Nhưng do giá thành quá cao, nó không thể thương mại hóa rộng rãi được.
Vào năm 1978, Storage Tek đã phát triển thành công một mẫu SSD đầu tiên. Giữa nhứng năm 1980, Santa Clara Systems giới thiệu BatRam, một kết hợp của các chip DIPRAM kèm theo một card điều khiển mô phỏng ổ cứng. Ngoài ra, BatRam còn kèm theo một pin hỗ trợ sạc nhiều lần để đảm bảo lưu trữ dữ liệu khi tắt nguồn. Phiên bản Sharp PC-5000, được giới thiệu năm 1983, sử dụng một ổ lưu trữ SSD 128Kb.
Năm 1996, M-system (được SanDisk mua lại năm 2006) giới thiệu một ổ SSD dựa trên công nghệ bộ nhớ flash. Kể từ thời điểm này, SSD được sử dụng như một lựa chọn thay thế cho HDD truyền thống trong các ngành công nghiệp hàng không vũ trụ , quân sự và những ngành nghiên cứu quan trọng khác. Những ứng dụng trong các ngành này đòi hỏi một thiết bị có khả năng lưu trữ bền vững ít lỗi vốn có trong thiết bị SSD.
SSD bắt đầu được sử dụng trong laptop mặc dù cho đến năm 2009, chi phí trên đơn vị lưu trữ của SSD vẫn đắt hơn HDD nhiều lần (580$ cho một ổ SSD 256GB, so với 50$ cho một ổ HDD sử dụng khe USB cắm ngoài có cùng dung lượng).
Tháng 3 năm 2009, Texas Memory System tuyên bố sử dụng hệ thống ổ SSD có dung lượng lớn nhất đạt tới 5TB có tên gọi RamSan-620 cho hệ thống lưu trữ dạng rack. Nó có khả năng đáp ứng tốc độ truy cập 3Gb/s và đáp ứng với tốc độ 250.000 thao tác đọc/ghi dữ liệu/giây (IOPS).
Tháng 5 năm 2009, Photofast giới thiệu ổ SSD G-Monster-PROMISE PCIe với dung lượng tùy chọn từ 128Gb đến 1TB, hỗ trợ tốc độ đọc ghi 1000MB/s.
DDR SDRAM based SSD. Max 128 GBand 3072 MB/s.
PCI attached IO Accelerator SSD
PCI-E, DRAM, and NAND based SSD
A solid-state drive (SSD) (sometimes improperly referred to as a “solid-state disk” or “electronic disk”) is a data storage device that uses integrated circuit assemblies as memory to store data persistently. SSD technology uses electronic interfaces compatible with traditional block input/output (I/O)hard disk drives. SSDs do not employ any moving mechanical components, which distinguishes them from traditional magnetic disks such as hard disk drives (HDDs) or floppy disks, which are electromechanical devices containing spinning disks and movable read/write heads. Compared with electromechanical disks, SSDs are typically less susceptible to physical shock, are silent, and have lower access time and latency, but are, at 2011 market prices, more expensive per unit of storage. The prices have continued to decline in 2012.
SSDs share the I/O interface technology developed for hard disk drives, thus permitting simple replacement for most applications.
As of 2010, most SSDs use NAND-based flash memory, which retains data without power. For applications requiring fast access, but not necessarily data persistence after power loss, SSDs may be constructed from random-access memory (RAM). Such devices may employ separate power sources, such as batteries, to maintain data after power loss.
Hybrid drives combine the features of SSDs and HDDs in the same unit, containing a large hard disk drive and an SSD cache to improve performance of frequently accessed data. These devices may offer near-SSD performance for many applications.
Development and history
Early SSDs using RAM and similar technology
The origins of SSDs came from the 1950s and used two similar technologies: magnetic core memory and card capacitor read-only store (CCROS). These auxiliary memory units, as they were called at the time, emerged during the era of vacuum tube computers. But with the introduction of cheaper drum storage units, their use was discontinued.
Later, in the 1970s and 1980s, SSDs were implemented in semiconductor memory for early supercomputers of IBM, Amdahl and Cray; however, the prohibitively high price of the built-to-order SSDs made them quite seldom used. In the late 1970s, General Instruments produced an electrically alterable ROM (EAROM) which operated somewhat like the later NAND flash memory. Unfortunately, a ten-year life was not achievable and many companies abandoned the technology. In 1976 Dataram started selling a product called Bulk Core, which provided up to 2 MB of solid state storage compatible with Digital Equipment Corporation (DEC) and Data General (DG) computers. In 1978, Texas Memory Systems introduced a 16 kilobyte RAM solid-state drive to be used by oil companies for seismic data acquisition. The following year, StorageTek developed the first modern type of solid-state drive.
The Sharp PC-5000, introduced in 1983, used 128 kilobyte solid-state storage cartridges, containing bubble memory. In 1984 Tallgrass Technologies Corporation had a tape back up unit of 40 MB with a solid state 20 MB unit built in. The 20 MB unit could be used instead of a hard drive. In September 1986, Santa Clara Systems introduced BatRam, a 4 megabytemass storage system expandable to 20 MB using 4 MB memory modules. The package included a rechargeable battery to preserve the memory chip contents when the array was not powered. 1987 saw the entry of EMC Corporation (EMC) into the SSD market, with drives introduced for the mini-computer market. However, by 1993 EMC had exited the SSD market.
Software-based RAM Disks are still used as of 2009 because they are an order of magnitude faster than the fastest SSD, but they consume CPU resources and cost much more on a per GB basis.
In 1994, STEC, Inc. bought Cirrus Logic’s flash controller operation, allowing the company to enter the flash memory business for consumer electronic devices.
In 1995, M-Systems introduced flash-based solid-state drives. They had the advantage of not requiring batteries to maintain the data in the memory (required by the prior volatile memory systems), but were not as fast as the DRAM-based solutions. Since then, SSDs have been used successfully as HDD replacements by the military and aerospace industries, as well as for other mission-critical applications. These applications require the exceptional mean time between failures (MTBF) rates that solid-state drives achieve, by virtue of their ability to withstand extreme shock, vibration and temperature ranges.
In 1999, BiTMICRO made a number of introductions and announcements about flash-based SSDs, including an 18 GB 3.5-inch SSD. In 2007, Fusion-io announced a PCIe-based SSD with 100,000 input/output operations per second (IOPS) of performance in a single card, with capacities up to 320 gigabytes. At Cebit 2009, OCZ Technology demonstrated a 1 terabyte (TB) flash SSD using a PCI Express ×8 interface. It achieved a maximum write speed of 654 megabytes per second (MB/s) and maximum read speed of 712 MB/s. In December 2009, Micron Technology announced the world’s first SSD using a 6 gigabits per second (Gbit/s) SATA interface.
Enterprise flash drives
Enterprise flash drives (EFDs) are designed for applications requiring high I/O performance (IOPS), reliability, and energy efficiency. In most cases, an EFD is an SSD with a higher set of specifications, compared with SSDs that would typically be used in notebook computers. The term was first used by EMC in January 2008, to help them identify SSD manufacturers who would provide products meeting these higher standards. There are no standards bodies who control the definition of EFDs, so any SSD manufacturer may claim to produce EFDs when they may not actually meet the requirements. Likewise, there may be other SSD manufacturers that meet the EFD requirements without being called EFDs.
Architecture and function
The key components of an SSD are the controller and the memory to store the data. The primary memory component in an SSD had been DRAM volatile memory since they were first developed, but since 2009 it is more commonly NAND flash non-volatile memory. Other components play a less significant role in the operation of the SSD and vary among manufacturers.
Every SSD includes a controller that incorporates the electronics that bridge the NAND memory components to the host computer. The controller is an embedded processor that executes firmware-level code and is one of the most important factors of SSD performance. Some of the functions performed by the controller include:
- Error correction (ECC)
- Wear leveling
- Bad block mapping
- Read scrubbing and read disturb management
- Read and write caching
- Garbage collection
The performance of an SSD can scale with the number of parallel NAND flash chips used in the device. A single NAND chip is relatively slow, due to narrow (8/16 bit) asynchronous IO interface, and additional high latency of basic I/O operations (typical for SLC NAND, ~25 μs to fetch a 4K page from the array to the I/O buffer on a read, ~250 μs to commit a 4K page from the IO buffer to the array on a write, ~2 ms to erase a 256 KiB block). When multiple NAND devices operate in parallel inside an SSD, the bandwidth scales, and the high latencies can be hidden, as long as enough outstanding operations are pending and the load is evenly distributed between devices. Micron and Intel initially made faster SSDs by implementing data striping (similar to RAID 0) and interleaving in their architecture. This enabled the creation of ultra-fast SSDs with 250 MB/s effective read/write speeds with the SATA 3 Gbit/s interface in 2009. Two years later, SandForce continued to leverage this parallel flash connectivity, releasing consumer-grade SATA 6 Gbit/s SSD controllers which supported 500 MB/s read/write speeds. SandForce controllers compress the data prior to sending it to the flash memory. This process may result in less writing and higher logical throughput, depending on the compressibility of the data.
Most SSD manufacturers use non-volatile NAND flash memory in the construction of their SSDs because of the lower cost compared with DRAM and the ability to retain the data without a constant power supply, ensuring data persistence through sudden power outages. Flash memory SSDs are slower than DRAM solutions, and some early designs were even slower than HDDs after continued use. This problem was resolved by controllers that came out in 2009 and later.
Flash memory-based solutions are typically packaged in standard disk drive form factors (1.8-, 2.5-, and 3.5-inch), or smaller unique and compact layouts because of the compact memory.
Lower priced drives usually use multi-level cell (MLC) flash memory, which is slower and less reliable than single-level cell (SLC) flash memory. This can be mitigated or even reversed by the internal design structure of the SSD, such as interleaving, changes to writing algorithms, and higher over-provisioning (more excess capacity) with which the wear-leveling algorithms can work.
SSDs based on volatile memory such as DRAM are characterized by ultrafast data access, generally less than 10 microseconds, and are used primarily to accelerate applications that would otherwise be held back by the latency of flash SSDs or traditional HDDs. DRAM-based SSDs usually incorporate either an internal battery or an external AC/DC adapter and backup storage systems to ensure data persistence while no power is being supplied to the drive from external sources. If power is lost, the battery provides power while all information is copied from random access memory (RAM) to back-up storage. When the power is restored, the information is copied back to the RAM from the back-up storage, and the SSD resumes normal operation (similar to the hibernate function used in modern operating systems).
SSDs of this type are usually fitted with DRAM modules of the same type used in regular PCs and servers, which can be swapped out and replaced by larger modules.
A remote, indirect memory-access disk (RIndMA Disk) uses a secondary computer with a fast network or (direct) Infiniband connection to act like a RAM-based SSD, but the new faster flash memory based SSDs already available in 2009 are making this option not as cost effective.
Cache or buffer
A flash-based SSD typically uses a small amount of DRAM as a cache, similar to the cache in hard disk drives. A directory of block placement and wear leveling data is also kept in the cache while the drive is operating. Data is not permanently stored in the cache. One SSD controller manufacturer, SandForce, does not use an external DRAM cache on their designs, but still achieves very high performance. Eliminating the external DRAM enables a smaller footprint for the other flash memory components in order to build even smaller SSDs.
Battery or super capacitor
Another component in higher performing SSDs is a capacitor or some form of battery. These are necessary to maintain data integrity such that 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. In the case of MLC flash memory, a problem called lower page corruption can occur when MLC flash memory loses power while programming an upper page. The result is that data written previously and presumed safe can be corrupted if the memory is not supported by a super capacitor in the event of a sudden power loss. This problem does not exist with SLC flash memory. Most consumer-class SSDs do not have built-in batteries or capacitors; among the exceptions are the Intel 320 series and the more expensive Intel 710 series.
The host interface is not specifically a component of the SSD, but it is a key part of the drive. The interface is usually incorporated into the controller discussed above. The interface is generally one of the interfaces found in HDDs. They include:
- Serial ATA
- Serial attached SCSI (generally found on servers)
- PCI Express
- Fibre Channel (almost exclusively found on servers)
- Parallel ATA (IDE) interface (mostly replaced by SATA)
- (Parallel) SCSI (generally found on servers; mostly replaced by SAS; last SCSI-based SSD introduced in 2004)
The size and shape of any device is largely driven by the size and shape of the components used to make that device. Traditional HDDs and optical drives are designed around the rotating platteror optical disc along with the spindle motor inside. If an SSD is made up of various interconnected integrated circuits (ICs) and an interface connector, then its shape could be virtually anything imaginable because it is no longer limited to the shape of rotating media drives. Some solid state storage solutions come in a larger chassis that may even be a rack-mount form factor with numerous SSDs inside. They would all connect to a common bus inside the chassis and connect outside the box with a single connector.
For general computer use, the 2.5″ form factor (typically found in laptops) is the most popular. For desktop computers with 3.5″ hard disk slots, a simple adapter plate can be used to make such disk fit. Other type of form factors are more common in enterprise applications. A SSD can also be completely integrated in the other circuitry of the device, as in the Apple MacBook Air (starting with the fall 2010 model).
Standard HDD form factors
The benefit of using a current HDD form factor would be to take advantage of the extensive infrastructure already in place to mount and connect the drives to the host system. These traditional form factors are known by the size of the rotating media, e.g., 5.25″, 3.5″, 2.5″, 1.8″, not by the dimensions of the drive casing.
Box form factors
Many of the DRAM-based solutions use a box that is often designed to fit in a rack-mount system. The number of DRAM components required to get sufficient capacity to store the data along with the backup power supplies requires a larger space than traditional HDD form factors.
Bare-board form factors
Viking Technology SATA Cube and AMP SATA Bridge multi-layer SSDs
Viking Technology SATADIMM based SSD
MO-297 SSD form factor
Custom connector SATA SSD
Form factors which were more common to memory modules are now being used by SSDs to take advantage of their flexibility in laying out the components. Some of these include PCIe, mini PCIe, mini-DIMM, MO-297, and many more. The SATADIMM from Viking Technology uses an empty DDR3 DIMM slot on the motherboard to provide power to the SSD with a separate SATA connector to provide the data connection back to the computer. The result is an easy-to-install SSD with a capacity equal to drives that typically take a full 2.5 in drive bay. At least one manufacturer, InnoDisk, is producing a drive that sits directly on the SATA connector on the motherboard without any other support or mechanical mount. Some SSDs are based on the PCIe form factor and connect both the data interface and power through the PCIe connector to the host. These drives can use either direct PCIe flash controllers or a PCIe-to-SATA bridge device which then connects to SATA flash controllers.
Ball grid array form factors
In the early 2000s, a few companies introduced SSDs in Ball Grid Array (BGA) form factors, such as M-Systems’ (now SanDisk) DiskOnChip and Silicon Storage Technology’s NANDrive(now produced by Greenliant Systems), and Memoright’s M1000 for use in embedded systems. The main benefits of BGA SSDs are their low power consumption, small chip package size to fit into compact subsystems, and that they can be soldered directly onto a system motherboard to reduce adverse effects from vibration and shock.
Comparison of SSD with hard disk drives
See also: Disk drive performance characteristics
SSD benchmark, showing about 230 MB/s reading speed, 210 MB/s writing speed and about 0.1 ms seek time, all independent from the accessed disk location.
Making a comparison between SSDs and ordinary (spinning) HDDs is difficult. Traditional HDD benchmarks tend to focus on the performance characteristics that are poor with HDDs, such as rotational latency and seek time. As SSDs do not need to spin or seek to locate data, they may prove vastly superior to HDDs in such tests. However, SSDs have challenges with mixed reads and writes, and their performance may degrade over time. SSD testing must start from the (in use) full disk, as the new and empty (fresh out of the box) disk may have much better write performance than it would show after only weeks of use.
Most of the advantages of solid-state disks over traditional hard drives are due to their ability to access data completely electronically instead of electromechanically. On the other hand, traditional hard drives offer significantly higher capacity for their price.
While SSDs appear to be more reliable than HDDs, researchers at the Center for Magnetic Recording Research “are adamant that today’s SSDs aren’t an order of magnitude more reliable than hard drives”. SSD failures are often catastrophic, with total data loss. While HDDs can fail in this manner as well, they often give warning that they are failing, allowing much or all of their data to be recovered.
Traditional hard drives store their data in a linear, ordered manner. SSDs, however, constantly rearrange their data while keeping track of their locations for the purpose of wear leveling. As such, 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.
If the computer can accept multiple hard disks, some people have found a combination of technologies to provide a good balance between price and storage space. This could mean, for example, storing the operating system on a small-capacity SSD and, other data (big applications, games, user data, etc.) on a large-capacity HDD.
The following table shows a detailed overview of the advantages and disadvantages of both technologies. Comparisons reflect typical characteristics, and may not hold for a specific device.
|Attribute or characteristic||Solid-state drive||Hard disk drive|
|Start-up time||Almost instantaneous; no mechanical components to prepare. May need a few milliseconds to come out of an automatic power-saving mode.||Disk spin-up may take several seconds. A system with many drives may need to stagger spin-up to limit peak power drawn, which is briefly high when an HDD is first started.|
|Random access time||About 0.1 ms – many times faster than HDDs because data is accessed directly from the flash memory||Ranges from 2.9 (high end server drive) to 12 ms (laptop HDD) due to the need to move the heads and wait for the data to rotate under the read/write head|
|Read latency time||Generally low because the data can be read directly from any location. In applications where hard disk seeks are the limiting factor, this results in faster boot and application launch times (see Amdahl’s law).||Generally high since the mechanical components require additional time to get aligned|
|Data transfer rate||SSD technology can deliver rather consistent read/write speed, but when lots of individual smaller blocks are accessed, performance is reduced. In consumer products the maximum transfer rate typically ranges from about 100 MB/s to 600 MB/s, depending on the disk. Enterprise market offers devices with multi-gigabyte per second throughput.||Once the head is positioned, when reading or writing a continuous track, an enterprise HDD can transfer data at about 140 MB/s. However accessing fragmented data implies a severe performance penalty. Data transfer rate depends also upon rotational speed, which can range from 4,200 to 15,000 rpm. and also upon the track (reading from the outer tracks is faster due higher absolute head velocity relative to the disk).|
|Consistent read performance||Read performance does not change based on where data is stored on an SSD||If data from different areas of the platter must be accessed, as with fragmented files, response times will be increased by the need to seek each fragment|
|Fragmentation||There is no benefit to reading data sequentially (beyond typical FS block sizes), making fragmentation irrelevant for SSDs. Defragmentation would cause wear by making additional writes of the NAND flash cells, which have a limited cycle life.||Files, particularly large ones, on HDDs usually become fragmented over time if frequently written; periodic defragmentation is required to maintain optimum performance.|
|Noise (acoustic)||SSDs have no moving parts and therefore are basically silent, although electric noise from the circuits may occur.||HDDs have moving parts (heads, actuator, and spindle motor) and make some sound; noise levels vary between models, but can be significant (while often much lower than the sound from the cooling fans).|
|Temperature control||SSDs do not usually require any special cooling and can tolerate higher temperatures than HDDs. High-end enterprise models delivered as add-on cards may be supplied fitted with heat sinks to dissipate heat generated.||According to Seagate, ambient temperatures above 95 °F (35 °C) can shorten the life of a hard disk, and reliability will be compromised at drive temperatures above 131 °F (55 °C). Fan cooling may be required if temperatures would otherwise exceed these values. In practice most hard drives are used without special arrangements for cooling.|
|High altitude operation||Can endure high altitude during operation.||Specially manufactured sealed and pressurized hard disks are needed for reliable high-altitude operation.|
|Susceptibility to environmental factors||No moving parts, very resistant to shock and vibration||Heads floating above rapidly rotating platters are susceptible to shock and vibration|
|Installation and mounting||Not sensitive to orientation, vibration, or shock. Usually no exposed circuitry.||Circuitry may be exposed, and must not contact metal parts. Most of recent models work well in all orientations. Should be mounted to protect against vibration and shock.|
|Susceptibility to magnetic fields ||No impact on flash memory||Magnets or magnetic surges could in principle damage data, although the magnetic platters are usually well-shielded inside a metal case.|
|Weight and size||Solid state drives, essentially semiconductor memory devices mounted on a circuit board, are small and light in weight. However, for easy replacement, they often follow the same form factors as HDDs (3.5″, 2.5″ or 1.8″). Such form factors typically weigh as much as their HDD counterparts, mostly due to the enclosure.||HDDs typically have the same form factor but may be heavier. This applies for 3.5″ drives, which typically weigh around 700 grams.|
|Reliability and lifetime||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. The controllers manage this limitation so that drives can last for many years under normal use. SSDs based on DRAM do not have a limited number of writes. Firmware bugs are currently a common cause for data loss.||HDDs have moving parts, and are subject to potential mechanical failures from the resulting wear and tear.|
|Secure writing limitations||NAND flash memory cannot be overwritten, but has to be rewritten to previously erased blocks. If a software encryption program encrypts data already on the SSD, the overwritten data is still unsecured, unencrypted, and accessible (drive-based hardware encryption does not have this problem). Also data cannot be securely erased by overwriting the original file without special “Secure Erase” procedures built into the drive.||HDDs can overwrite data directly on the drive in any particular sector.|
|Cost per capacity||NAND flash SSDs cost approximately US$0.65 per GB ||HDDs cost about US$0.05 per GB for 3.5 inch and $0.10 per GB for 2.5 inch drives|
|Storage capacity||In 2011 SSDs were available in sizes up to 2 TB, but less costly 64 to 256 GB drives were more common.||In 2011 HDDs of up to 4 TB were available.|
|Read/write performance symmetry||Less expensive SSDs typically have write speeds significantly lower than their read speeds. Higher performing SSDs have similar read and write speeds.||HDDs generally have slightly lower write speeds than their read speeds.|
|Free block availability and TRIM||SSD write performance is significantly impacted by the availability of free, programmable blocks. Previously written data blocks no longer in use can be reclaimed by TRIM; however, even with TRIM, fewer free blocks cause slower performance.||HDDs are not affected by free blocks and do not benefit from TRIM|
|Power consumption||High performance flash-based SSDs generally require half to a third of the power of HDDs. High-performance DRAM SSDs generally require as much power as HDDs, and must be connected to power even when the rest of the system is shut down.||The lowest-power HDDs (1.8″ size) can use as little as 0.35 watts. 2.5″ drives typically use 2 to 5 watts. The highest-performance 3.5″ drives can use up to about 20 watts.|
Comparison of SSDs with memory cards
CompactFlash card used as an SSD
While both memory cards and most SSDs use flash memory, they serve very different markets and purposes. Each has a number of different attributes which are optimized and adjusted to best meet the needs of particular users. Some of these characteristics include power consumption, performance, size, and reliability.
SSDs were originally designed for use in a computer system. The first units were intended to replace or augment hard disk drives, so the operating system recognized them as a hard drive. Originally, solid state drives were even shaped and mounted in the computer like hard drives. Later SSDs became smaller and more compact, eventually developing their own unique form factors. The SSD was designed to be installed permanently inside a computer.
In contrast, memory cards (such as Secure Digital (SD), CompactFlash and many others) were originally designed for digital cameras and later found their way into cell phones, gaming devices, GPS units, etc. Most memory cards are physically smaller than SSDs, and designed to be inserted and removed repeatedly. There are adapters which enable some memory cards to interface to a computer, allowing use as an SSD, but they are not intended to be the primary storage device in the computer. The typical CF card interface is generally 3-4 times slower than an SSD. As memory cards are not designed to tolerate the amount of reading and writing which occurs during typical computer use, their data may get damaged unless special procedures are taken to reduce the wear on the card to a minimum.
Until 2009, SSDs were mainly used in those aspects of mission critical applications where the speed of the storage system needed to be as fast as possible. Since flash memory has become a common component of SSDs, the falling prices and increased densities have made it more financially attractive for many other applications. Organizations that can benefit from faster access of system data include equity trading companies, telecommunication corporations, streaming media and video editing firms. The list of applications which could benefit from faster storage is vast. Any company can assess the ROI from adding SSDs to their own applications to best understand if that will be cost effective for them.
Flash-based solid-state drives can be used to create network appliances from general-purpose personal computer hardware. A write protected flash drive containing the operating system and application software can substitute for larger, less reliable disk drives or CD-ROMs. Appliances built this way can provide an inexpensive alternative to expensive router and firewall hardware.
SSDs based on an SD card with a live SD operating system are easily write-locked. Combined with a cloud computing environment or other writable medium, to maintain persistence, an OSbooted from a write-locked SD card is robust, rugged, reliable, and impervious to permanent corruption. If the running OS degrades, simply turning the machine off and then on returns it back to its initial uncorrupted state and thus is particularly solid. The SD card installed OS does not require removal of corrupted components since it was write-locked though any written media may need to be restored.
In 2011, Intel introduced a caching mechanism for their Z68 chipset (and mobile derivatives) called Smart Response Technology, which allows a SATA SSD to be used as a cache (configurable as write-through or write-back) for a conventional, magnetic hard disk drive. A similar technology is available on HighPoint’s RocketHybrid PCIe card. Hybrid drives (H-HDSs) are based on the same principle, but integrate some amount of flash memory on board of a conventional drive instead of using a separate SSD. The flash layer in these drives can be accessed independently from the magnetic storage by the host using ATA-8 commands, allowing the operating system to manage it. For example Microsoft’s ReadyDrive technology explicitly stores portions of the hibernation file in the cache of these drives when the system hibernates, making the subsequent resume faster.
Data recovery and secure deletion
Solid state drives have set new challenges for data recovery companies, as the way of storing data is much more non-linear and complex than of hard disk drives. The strategy the drive operates by internally can largely vary between manufacturers and, the TRIM command zeroes the whole range of a deleted file. Wear leveling also means that the physical and virtual location of data pieces differ.
As for secure deletion of data, using the ATA Secure Erase command is recommended, as the drive itself knows the most effective method to truly reset its data. A program such as Parted Magiccan be used for this purpose.
SSD-optimized file systems
Usually the same file systems used on hard disk drives can also be used on solid state disks. However it is recommended that file system supports the TRIM command which helps the SSD to recycle discarded data. There is no need for the file system to take care of wear leveling or other flash memory characteristics, as they are handled internally by the SSD.
While not a file system feature, operating systems must also align partitions correctly to avoid excessive read-modify-write cycles. Other features designed for hard disk drives, most notablydefragmentation, are disabled in SSD installations.
Listed below are some notable computer file systems which are optimized for solid-states drives.
The Linux kernel supports the TRIM function starting with version 2.6.33. The ext4 and Btrfs (experimental) file systems are supported when mounted using the discard parameter. Linux distributions usually do not set this kind of configuration automatically during installation. This is because of the notion that the operating system might after all not use the disk optimally when configured as such. The disk utilities take care of proper partition alignment.
Mac OS X
Mac OS X 10.7 (Lion) supports TRIM, as does OS X 10.6.8 Snow Leopard. There is also a technique to enable TRIM in earlier versions, though it is uncertain whether TRIM is utilized properly if enabled in versions before 10.6.8.
Windows 7Versions of Microsoft Windows prior to Vista do not take any special measures to support solid state drives. Partitions can be manually aligned before OS installation. Defragmentation negatively affects the life of the SSD and has no benefit. The TRIM command can be triggered using third-party tools to help maintain performance over time.
Windows 7 has support for SSDs. The operating system detects the presence of an SSD and optimizes operation accordingly. For SSD devices Windows 7 disables defragmentation,Superfetch and ReadyBoost, which are boot-time and application prefetching operations. It also includes support for the TRIM command to reduce garbage collection for data which the operating system has already determined is no longer valid. Without support for TRIM, the SSD would be unaware of this data being invalid and would unnecessarily continue to rewrite it during garbage collection causing further wear on the SSD.
Windows Vista generally expects hard disk drives rather than SSDs. Windows Vista includes ReadyBoost to exploit characteristics of USB-connected flash devices, but for SSDs it only improves the default partition alignment to prevent read-modify-write operations which reduce the speed of the SSD. This is because most SSDs are typically aligned on 4 KB sectors and most systems are based on 512 byte sectors with the default partition set up unaligned . The proper alignment really does not help the SSD’s endurance over the life of the drive, however some Vista operations, if not disabled, can shorten the life of the SSD. Disk defragmentation should be disabled because the location of the file components on an SSD doesn’t significantly impact its performance, but moving the files to make them contiguous using the Windows Defrag routine will cause unnecessary write wear on the limited number of P/E cycles on the SSD. The Superfetchfeature will not materially improve the performance of the system and causes additional overhead in the system and SSD, although it does not cause wear. Vista does not natively implement TRIM command, either.
Solaris as of version 10 Update 6 (released in October 2008), and recent versions of OpenSolaris, Solaris Express Community Edition, Illumos, Linux with ZFS on Linux and FreeBSD all can use SSDs as a performance booster for ZFS. A low-latency SSD can be used for the ZFS Intent Log (ZIL), where it is named the SLOG. This is used every time a synchronous write to the disk occurs. An SSD (not necessarily with a low-latency) may also be used for the level 2 Adaptive Replacement Cache (L2ARC), which is used to cache data for reading. When used either alone or in combination, large increases in performance are generally seen.
In addition to the ZFS features described above, the Unix File System (UFS) supports the TRIM command.
On Linux, swap partitions automatically exploit TRIM operations when the underlying drive supports TRIM (no configuration is needed). On some operating systems[which?], there might not be a possibility to use the TRIM function on discrete swap partitions. To remedy this issue, swap files inside an ordinary file system may be used.
DragonFly BSD allows SSD-configured swap to also be used as file system cache. This can be used to boost performance on both desktop and server workloads. The bcache project provides a similar concept for the Linux kernel.
The following are noted standardization organizations and bodies that work to create standards for solid-state drives (and other computer storage devices). The table below also includes organizations who promote the use of solid-state drives. This is not necessarily an exhaustive list.
|Organization or Committee||Subcommittee of:||Purpose|
|INCITS||N/A||Coordinates technical standards activity between ANSI in the USA and joint ISO/IEC committees worldwide|
|JEDEC||N/A||Develops open standards and publications for the microelectronics industry|
|JC-64.8||JEDEC||Focuses on solid-state drive standards and publications|
|NVMHCI||N/A||Provides standard software and hardware programming interfaces for nonvolatile memory subsystems|
|SATA-IO||N/A||Provides the industry with guidance and support for implementing the SATA specification|
|SFF Committee||N/A||Works on storage industry standards needing attention when not addressed by other standards committees|
|SNIA||N/A||Develops and promotes standards, technologies, and educational services in the management of information|
|SSSI||SNIA||Fosters the growth and success of solid state storage|
Cost and capacity
The technological trend of 50% decline in costs per year is no longer possible in NAND flash due to patents on some key manufacturing processes stifling further competition in the market. Due to this, most current NAND makers anticipate modest cost declines in the period between 2011-2015. Capacities in client SSDs are typically dictated by cost concerns rather than technical limitations of NAND storage.. The cost per gigabyte tends to increase up to several times for the top capacity drives but this may also be dictated by marketing decisions.
Solid-state drive technology has been marketed to the military and niche industrial markets since the mid-1990s..
Along with the emerging enterprise market, SSDs have been appearing in ultra-mobile PCs and a few lightweight laptop systems, adding significantly to the price of the laptop, depending on the capacity, form factor and transfer speeds. For low-end applications, a USB flash drive may be obtainable for anywhere from $10 to $100 or so, depending on capacity; alternatively, aCompactFlash card may be paired with a CF-to-IDE or CF-to-SATA converter at a similar cost. Either of these requires that write-cycle endurance issues be managed, either by refraining from storing frequently written files on the drive or by using a flash file system. Standard CompactFlash cards usually have write speeds of 7 to 15 MB/s while the more expensive upmarket cards claim speeds of up to 60 MB/s.
One of the first mainstream releases of SSD was the XO Laptop, built as part of the One Laptop Per Child project. Mass production of these computers, built for children in developing countries, began in December 2007. These machines use 1,024 MiB SLC NAND flash as primary storage which is considered more suitable for the harsher than normal conditions in which they are expected to be used. Dell began shipping ultra-portable laptops with SanDisk SSDs on April 26, 2007. Asus released the Eee PC subnotebook on October 16, 2007, with 2, 4 or 8 gigabytes of flash memory. On January 31, 2008, Apple released the MacBook Air, a thin laptop with an optional 64 GB SSD. The Apple Store cost was $999 more for this option, as compared with that of an 80 GB 4200 RPM hard disk drive. Another option, the Lenovo ThinkPad X300 with a 64 gigabyte SSD, was announced by Lenovo in February 2008. On August 26, 2008, Lenovo released ThinkPad X301 with 128 GB SSD option which adds approximately $200 US.
The Mtron SSD
In 2008 low end netbooks appeared with SSDs. In 2009 SSDs began to appear in laptops.
On January 14, 2008, EMC Corporation (EMC) became the first enterprise storage vendor to ship flash-based SSDs into its product portfolio.
In 2008 Sun released the Sun Storage 7000 Unified Storage Systems (codenamed Amber Road), which use both solid state drives and conventional hard drives to take advantage of the speed offered by SSDs and the economy and capacity offered by conventional hard disks.
Dell began to offer optional 256 GB solid state drives on select notebook models in January 2009.
In May 2009, Toshiba launched a laptop with a 512 GB SSD.
Since October 2010, Apple’s MacBook Air line has used a solid state drive as standard.
In December 2010, OCZ RevoDrive X2 PCIe SSD was available in 100 GB to 960 GB capacities delivering speeds over 740 MB/s sequential speeds and random small file writes up to 120,000 IOPS. 
In November 2010, Fusion-io released its highest performing SSD drive named ioDrive Octal utilising PCI-Express x16 Gen 2.0 interface with storage space of 5.12 TB, read speed of 6.0 GB/s, write speed of 4.4 GB/s and a low latency of 30 microseconds. It has 1.19 M Read 512 byte IOPS and 1.18 M Write 512 byte IOPS.
In 2011, computers based on Intel’s Ultrabook specifications became available. These specifications dictate that Ultrabooks use an SSD. These are consumer-level devices (unlike many previous flash offerings aimed at enterprise users), and represent the first widely available consumer computers using SSDs aside from the Macbook Air.
At CES 2012, OCZ Technology demonstrated the R4 CloudServ PCIe SSDs capable of reaching transfer speeds of 6.5 GB/s and 1.4 million IOPS. Also announced was the Z-Drive R5 which is available in capacities up to 12 TB capable of reaching transfer speeds of 7.2 GB/s and 2.52 million IOPS using the PCI Express x16 Gen 3.0
Quality and performance
SSD technology has been developing rapidly. Most of the performance measurements used on disk drives with rotating media are also used on SSDs. Performance of flash-based SSDs is difficult to benchmark because of the wide range of possible conditions. In a test performed in 2010 by Xssist, using IOmeter, 4 KB random 70% read/30% write, queue depth 4, the IOPS delivered by the Intel X25-E 64 GB G1 started around 10,000 IOPs, and dropped sharply after 8 minutes to 4,000 IOPS, and continued to decrease gradually for the next 42 minutes. IOPS vary between 3,000 to 4,000 from around 50 minutes onwards for the rest of the 8+ hours test run.
Write amplification is the major reason for the change in performance of an SSD over time. Designers of enterprise-grade drives try to avoid this performance variation by increasing over provisioning, and by employing wear-leveling algorithms that move data only when the drives are not heavily utilized.
Theo : wikipedia