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Solid State Drives
Update Time: 2017/5/18
solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies as memory to store data persistently. SSD technology primarily uses electronic interfaces compatible with traditionalblock input/output (I/O) hard disk drives (HDDs), which permit simple replacements in common applications. New I/O interfaces like SATA Expressand M.2 have been designed to address specific requirements of the SSD technology.

SSDs have no moving mechanical components. This distinguishes them from traditional electromechanical magnetic disks such as hard disk drives (HDDs) or floppy disks, which contain spinning disks and movable read/write heads. Compared with electromechanical disks, SSDs are typically more resistant to physical shock, run silently, and have lower access time and lower latency. However, while the price of SSDs has continued to decline over time (24 cents per gb as of 2017), consumer-grade SSDs are (as of 2017) still roughly four times more expensive per unit of storage than consumer-grade HDDs.

As of 2015, most SSDs use MLC NAND-based flash memory, which is a type ofnon-volatile memory that retains data when power is lost. 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 batteries as integrated power sources to retain data for a certain amount of time after external power is lost.

Hybrid drives or solid-state hybrid drives (SSHDs) 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.

ttribute 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.[113]
Random accesstime[114] Typically under 0.1 ms.[115] As data can be retrieved directly from various locations of the flash memory, access time is usually not a big performance bottleneck. 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 rotateunder the read/write head.[116]
Read latency time[117] 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).[118] Much higher than SSDs. Read time is different for every different seek, since the location of the data on the disk and the location of the read-head make a difference.
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 200 MB/s to 2500 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, a modern HDD can transfer data at about 200 MB/s. In practice transfer speeds are many times lower due to constant seeking, as files are read from various locations or they are fragmented. Data transfer rate depends also upon rotational speed, which can range from 3,600 to 15,000 rpm[119] and also upon the track (reading from the outer tracks is faster).
Read performance[120] Read performance does not change based on where data is stored on an SSD.[113]

Unlike mechanical hard drives, current SSD technology suffers from a performance degradation phenomenon called write amplification, where the NAND cells show a measurable drop in performance, and will continue degrading throughout the life of the SSD.[121] A technique called wear leveling is implemented to mitigate this effect, but due to the nature of the NAND chips, the drive will inevitably degrade at a noticeable rate.

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.[122]
Impacts of file system fragmentation There is limited benefit to reading data sequentially (beyond typical FS block sizes, say 4 KB), making fragmentation negligible for SSDs. Defragmentation would cause wear by making additional writes of the NAND flash cells, which have a limited cycle life.[123][124] However, even on SSDs there is a practical limit on how much fragmentation certain file systems can sustain; once that limit is reached, subsequent file allocations fail.[125]Consequently, defragmentation may still be necessary, although to a lesser degree.[125] Most file systems become fragmented over time if frequently written; periodic defragmentation is required to maintain optimum performance.[126] This usually is not an issue in modern file systems.
Noise (acoustic)[127] SSDs have no moving parts and therefore are basically silent, although on some low-grade SSDs, high pitch noise from the high voltage generator (for erasing blocks) may occur. HDDs have moving parts (headsactuator, and spindle motor) and make characteristic sounds of whirring and clicking; noise levels vary between models, but can be significant (while often much lower than the sound from the cooling fans). Laptop hard disks are relatively quiet.
Temperature control[128] A research conducted by Facebook found a consistent failure rate at temperatures between 30 to 40 °C. Failure rate rises when operating at temperatures higher than 40 °C, further increase of temperature may trigger thermal throttling around 70 °C, resulting reduced runtime performance. Reliability of early SSDs without thermal throttling are more affected by temperature, than newer ones with thermal throttling.[129] In practice, SSDs usually do not require any special cooling and can tolerate higher temperatures than HDDs. High-end enterprise models installed as add-on cards or 2.5-inch bay devices may ship with heat sinks to dissipate generated heat, requiring certain volumes of airflow to operate.[130] Ambient temperatures above 35 °C (95 °F) can shorten the life of a hard disk, and reliability will be compromised at drive temperatures above 55 °C (131 °F). Fan cooling may be required if temperatures would otherwise exceed these values.[131] In practice, modern HDDs may be used with no special arrangements for cooling.
Lowest operating temperature[132] SSDs can operate at −55 °C (−67 °F). Most modern HDDs can operate at 0 °C (32 °F).
Highest altitude when operating[133] SSDs have no issues on this.[134] HDDs can operate safely at an altitude of at most 3,000 meters (10,000 ft). HDDs will fail to operate at altitudes above 12,000 meters (40,000 ft).[135] With the introduction of Nitrogen Filled[citation needed] (sealed) HDDs, this is expected to be less of an issue.
Moving from a cold environment to a warmer environment SSDs have no issues on this.[citation needed] A certain amount of acclimation time is needed when moving HDDs from a cold environment to a warmer environment prior to operating it; otherwise, internal condensation will occur and operating it immediately will result in damage to its internal components.[136]
Breather hole SSDs do not require breather hole. Most modern HDDs require breather hole in order for it to function properly.[135]
Susceptibility toenvironmental factors[118][137][138] No moving parts, very resistant to shock, vibration, and movement. Heads floating above rapidly rotating platters are susceptible to shock, vibration, and movement.
Installation and mounting Not sensitive to orientation, vibration, or shock. Usually no exposed circuitry. Circuitry may be exposed, and it must not be short-circuited by conductive materials (such as the metal chassis of a computer). Should be mounted to protect against vibration and shock. Some HDDs should not be installed in a tilted position.[139]
Susceptibility tomagnetic fields Low impact on flash memory, but anelectromagnetic pulse will damage any electrical system, especially integrated circuits. In general, magnets or magnetic surges may result in data corruption or mechanical damage to the drive internals. Drive's metal case provides a low level of shielding to the magnetic platters.[140][141][142]
Weight and size[137] SSDs, essentially semiconductor memory devices mounted on a circuit board, are small and lightweight. They often follow the same form factors as HDDs (2.5-inch or 1.8-inch), but the enclosures are made mostly of plastic. HDDs are generally heavier than SSDs, as the enclosures are made mostly of metal, and they contain heavy objects such as motors and large magnets. 3.5-inch drives 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.[143][144][145][146][147] SSDs based on DRAM do not have a limited number of writes. However the failure of a controller can make a SSD unusable. Reliability varies significantly across different SSD manufacturers and models with return rates reaching 40% for specific drives.[107] As of 2011 leading SSDs have lower return rates than mechanical drives.[105] 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.[148][needs update?] HDDs have moving parts, and are subject to potential mechanical failures from the resulting wear and tear. The storage medium itself (magnetic platter) does not essentially degrade from read and write operations.

According to a study performed by Carnegie Mellon University for both consumer and enterprise-grade HDDs, their average failure rate is 6 years, and life expectancy is 9–11 years.[149] Leading SSDs have overtaken hard disks for reliability,[105] however the risk of a sudden, catastrophic data loss can be lower for mechanical disks.[150]

When stored offline (unpowered in shelf) in long term, the magnetic medium of HDD retains data significantly longer than flash memory used in SSDs.

Secure writing limitations NAND flash memory cannot be overwritten, but has to be rewritten to previously erased blocks. If a software encryptionprogram 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.[151] HDDs can overwrite data directly on the drive in any particular sector. However, the drive's firmware may exchange damaged blocks with spare areas, so bits and pieces may still be present. Most HDD manufacturers offer a tool that can zero-fill all sectors, including the reallocated ones.[citation needed]
Cost per capacity SSD pricing changes rapidly: US$0.59 per GB in April 2013,[152] US$0.45 per GB in April 2014, and US$0.37 per GB in February 2015,[153] about US$0.23 per GB in September 2016.[154] HDDs cost about US$0.03 per GB for 3.5-inch and US$0.04 per GB for 2.5-inch drives in September 2016.[154]
Storage capacity In 2016, SSDs were available in sizes up to 60 TB,[155] but less costly, 120 to 512 GB models were more common. In 2016, HDDs of up to 14 TB[156] 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 longer (worse) seek times for writing than for reading.[157]
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.[41][158][159] 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.[160][161] Emerging technologies like DevSlp can minimize power requirements of idle drives. The lowest-power HDDs (1.8-inch size) can use as little as 0.35 watts when idle.[162] 2.5-inch drives typically use 2 to 5 watts. The highest-performance 3.5-inch drives can use up to about 20 watts.
Maximum areal storage density (Terabits per square inch) 2.8 1.5

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