In comparison to hard disk drives and similar electromechanical media which use moving parts, SSDs are typically more resistant to physical shock, run silently, and have higher input/output rates and lower latency.[5] SSDs store data in semiconductor cells. As of 2019,[update] cells can contain between 1 and 4 bits of data. SSD storage devices vary in their properties according to the number of bits stored in each cell, with single-bit cells ("Single Level Cells" or "SLC") being generally the most reliable, durable, fast, and expensive type, compared with 2- and 3-bit cells ("Multi-Level Cells/MLC" and "Triple-Level Cells/TLC"), and finally quad-bit cells ("QLC") being used for consumer devices that do not require such extreme properties and are the cheapest per gigabyte (GB) of the four. In addition, 3D XPoint memory (sold by Intel under the Optane brand) stores data by changing the electrical resistance of cells instead of storing electrical charges in cells, and SSDs made from RAM can be used for high speed, when data persistence after power loss is not required, or may use battery power to retain data when its usual power source is unavailable.[6] Hybrid drives or solid-state hybrid drives (SSHDs), such as Intel's Hystor[7] and Apple's Fusion Drive, combine features of SSDs and HDDs in the same unit using both flash memory and spinning magnetic disks in order to improve the performance of frequently-accessed data.[8][9][10] Bcache achieves a similar effect purely in software, using combinations of dedicated regular SSDs and HDDs.
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SSDs based on NAND flash will slowly leak charge over time if left for long periods without power. This causes worn-out drives (that have exceeded their endurance rating) to start losing data typically after one year (if stored at 30 C) to two years (at 25 C) in storage; for new drives it takes longer.[11] Therefore, SSDs are not suitable for archival storage. 3D XPoint is a possible exception to this rule; it is a relatively new technology with unknown long-term data-retention characteristics
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).[99][100]
In 2015, Intel and Micron announced 3D XPoint as a new non-volatile memory technology.[106] Intel released the first 3D XPoint-based drive (branded as Intel Optane SSD) in March 2017 starting with a data center product, Intel Optane SSD DC P4800X Series, and following with the client version, Intel Optane SSD 900P Series, in October 2017. Both products operate faster and with higher endurance than NAND-based SSDs, while the areal density is comparable at 128 gigabits per chip.[107][108][109][110] For the price per bit, 3D XPoint is more expensive than NAND, but cheaper than DRAM.[111][self-published source?]
Drives known as hybrid drives or solid-state hybrid drives (SSHDs) use a hybrid of spinning disks and flash memory.[114][115] Some SSDs use magnetoresistive random-access memory (MRAM) for storing data.[116][117]
A flash-based SSD typically uses a small amount of DRAM as a volatile cache, similar to the buffers in hard disk drives. A directory of block placement and wear leveling data is also kept in the cache while the drive is operating.[83] One SSD controller manufacturer, SandForce, does not use an external DRAM cache on their designs but still achieves high performance. Such an elimination of the external DRAM reduces the power consumption and enables further size reduction of SSDs.[118]
Another component in higher-performing SSDs is a capacitor or some form of battery, which are necessary to maintain data integrity so the data in the cache can be flushed to the drive when power is lost; some may even hold power long enough to maintain data in the cache until power is resumed.[118][119] 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 supercapacitor in the event of a sudden power loss. This problem does not exist with SLC flash memory.[81]
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.[137] 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-inch drive bay.[138] At least one manufacturer, Innodisk, has produced a drive that sits directly on the SATA connector (SATADOM) on the motherboard without any need for a power cable.[139] 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[140] or a PCIe-to-SATA bridge device which then connects to SATA flash controllers.[141]
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 drive, as the new and empty (fresh, out-of-the-box) drive may have much better write performance than it would show after only weeks of use.[147]
Most of the advantages of solid-state drives over traditional hard drives are due to their ability to access data completely electronically instead of electromechanically, resulting in superior transfer speeds and mechanical ruggedness.[148] On the other hand, hard disk drives offer significantly higher capacity for their price.[5][149]
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.[162][163][164][165][166] SSDs based on DRAM do not have a limited number of writes. However the failure of a controller can make an SSD unusable. Reliability varies significantly across different SSD manufacturers and models with return rates reaching 40% for specific drives.[151] Many SSDs critically fail on power outages; a December 2013 survey of many SSDs found that only some of them are able to survive multiple power outages.[167][needs update?]A Facebook study found that sparse data layout across an SSD's physical address space (e.g., non-contiguously allocated data), dense data layout (e.g., contiguous data) and higher operating temperature (which correlates with the power used to transmit data) each lead to increased failure rates among SSDs.[168]
SSDs have very different failure modes from traditional magnetic hard drives. Because solid-state drives contain no moving parts, they are generally not subject to mechanical failures. Instead, other kinds of failure are possible (for example, incomplete or failed writes due to sudden power failure can be more of a problem than with HDDs, and if a chip fails then all the data on it is lost, a scenario not applicable to magnetic drives). On the whole, however, studies have shown that SSDs are generally highly reliable, and often continue working far beyond the expected lifetime as stated by their manufacturer.[219]
A 2016 field study based on data collected over six years in Google's data centres and spanning "millions" of drive days found that the proportion of flash-based SSDs requiring replacement in their first four years of use ranged from 4% to 10% depending on the model. The authors concluded that SSDs fail at a significantly lower rate than hard disk drives.[219] (In contrast, a 2016 evaluation of 71,940 HDDs found failure rates comparable to those of Google's SSDs: the HDDs had on average an annualized failure rate of 1.95%.)[223] The study also showed, on the down-side, that SSDs experience significantly higher rates of uncorrectable errors (which cause data loss) than do HDDs. It also led to some unexpected results and implications:
Solid-state drives have set new challenges for data recovery companies, as the method of storing data is non-linear and much more complex than that of hard disk drives. The strategy by which the drive operates internally can vary largely between manufacturers, and the TRIM command zeroes the whole range of a deleted file. Wear leveling also means that the physical address of the data and the address exposed to the operating system are different.
Typically the same file systems used on hard disk drives can also be used on solid state drives. It is usually expected for the file system to support the TRIM command which helps the SSD to recycle discarded data (support for TRIM arrived some years after SSDs themselves but is now nearly universal). This means that the file system does not need to manage wear leveling or other flash memory characteristics, as they are handled internally by the SSD. Some log-structured file systems (e.g. F2FS, JFFS2) help to reduce write amplification on SSDs, especially in situations where only very small amounts of data are changed, such as when updating file-system metadata. 2ff7e9595c
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