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Introduction to Solid State Drives (SSD)


Introduction to Solid State Drives (SSD)

Introduction to Solid State Drives (SSD)

Introduction to Solid State Drives (SSD) NNuFKyB

The SSD (Solid State Drive) 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 built with no moving parts and are typically more resistant to physical shock, run silently, have lower access time, wont slurp up batteries  and lower latency. However, currently a solid state drive is over four times more expensive than the best hard disk drive. Although SSDs are still more expensive per gigabytes (GB) than traditional hard disk drives, the price difference is becoming smaller as their capacities have simultaneously increased. SSDs are now widely used for applications where cost is not as important as performance and durability. Tablets, smartphones, netbooks, and Ultrabooks use SSDs.

SSDs use NAND flash memory technology. 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. Three subtypes of this technology are in used, and commercially available, most mainstream SSDs use MLC (multilevel cell) or TLC (triple-levelcell). As of 2015, most SSDs use MLC NAND based flash memory. MLC stores 2 bits in a single cell whereas TLC, stores 3 bits in a single cell. Specialized high-end products mostly for server or workstation systems use SLC (single-level cell), flash memory stores data in individual memory cells, SLC flash stores 1 bit in a single cell with SLC versions of SSD offering higher performance, lower capacity, and higher cost. The SLC is the fastest for read and erase functions, followed by MLC, with TLC bringing up the rear. The TLC has slow down access time, also has a decrease expected life of the SSD memory cells. One potential problem with flash memory is that it wears out. SLC flash cells are normally rated for 100,000  Program-Erase (PE) cycles, whereas MLC flash cells are normally rated for only 1,000 to 10,000 Program-Erase (PE) cycles, and TLC drives for only about 1,000  Program-Erase (PE) cycles. To mitigate this wear, SSDs incorporate sophisticated wear-leveling algorithms that essentially vary or rotate the usage of cells so that no single cell or group of cells is used more than another. In addition, spare cells are provided to replace those that do wear out, thus extending the life of the drive. Although the differences in lifespan of different for each NAND flash cell memory types, estimates based on actual workload tests suggest that even TLC drives can last more than 10 years. 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. Reliability varies significantly across different SSD manufacturers and models with return rates reaching 40% for specific drives. As of 2011 leading SSDs have lower return rates than mechanical drives. Many SSDs critically fail on power outages; a  survey of many SSDs found that only some of them are able to survive multiple power outages. Failure of a controller can make a SSD unusable.

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. On the other hand, hard disk drives offer significantly higher capacity for their price and have longer backup storage life.

While both memory cards and most SSDs use flash memory, they serve very different markets and purposes. 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. In contrast, memory cards (such as Secure Digital (SD), CompactFlash (CF), and many others) were originally designed for digital cameras and later found their way into cell phones, gaming devices, GPS units, etc. 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 CompactFlash card interface is three to four 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.

Gamers want there games to load instantly and play at maximum possible frames per second (FPS) with all features turned on. Using a solid state drive will give the gamer the extra edge he or she is seeking. The system will be in general more responsive and although the latency of the HDDs initial seek is short, the absence of it tended to make the system feel faster.

In 2015, SSDs were available in sizes up to 16 TB, but less costly, 120 to 512 GB models were more common.

SSD random access time typically under 0.1 ms. As data can be retrieved directly from various locations of the flash memory, access time is usually not a big performance bottleneck. Where as HDD random access time 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.

With SSD drives the read latency time generally very low because the data can be read directly from any location.  The HDD read latency time much higher than SSDs.

SSDs usually do not require any special cooling and can tolerate higher temperatures than HDDs. With HDDs the 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).

With SSD drive the operating systems boot up time is much more faster than hard disk drive.

A flash-based SSD typically use 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.

Drives known as hybrid drives or solid-state hybrid drives (SSHDs) use a hybrid of spinning disks and flash memory. Solid-state hybrid drives (SSHDs) 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. 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.

Because of the way SSDs work internally, the concept of file fragmentation is immaterial, and running a defragmenting program on an SSD does nothing except cause it to wear out sooner. Unlike magnetic drives, which must move the heads to access data written to different physical areas of the disk, an SSD can read data from different areas of memory without delay. SSDs should not be defragmented like traditional magnetic drives.
What was previously known as the Windows Disk Defragmenter, has now been converted into optimize SSD drive utility in Windows 8 and later versions of Windows OS. With Windows 8 and later versions of Windows OS the Windows Disk Defragmenter now addresses fragmentation on hard disk drive, as well performs TRIM cleanup on solid state drives. The Windows Disk Defragmenter will automatically detects storage types that are connected, and will perform defragmentation on HDDs, or send TRIM commands to SSDs.

Windows 7 and later operating systems are SSD aware, which means they can tell an SSD from a standard magnetic drive. When Windows 7 detects that an SSD is attached, it automatically turns off the background Disk Defragmenter function, thus preserving drive endurance. When using SSDs with Windows Vista and earlier versions, you should manually disable Disk Defragmenter or otherwise prevent any form of defragmentation program or operation from running on SSDs.

Unlike spinning media, Flash media can not simply overwrite existing data and must first perform a 512KB block erase. To overwrite existing data, an SSD requires a longer read-modify-write operation which is the basis for slowing write performance as the drive fills. A common misconception is that discarded blocks of an SSD drive are immediately erased. This is not usually the case. Instead, the way the TRIM command operates is considering the contents of discarded blocks as indeterminate (the "don't care" state) until the moment these blocks are physically erased by a separate background process, the garbage collector. In other words, the TRIM command does not erase the content of discarded blocks by itself. Instead, it adds them to a queue of pending blocks for being cleared by the garbage collector at time of  when the drives are not heavily utilize.

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.

Deleted files and data may be lost forever in a matter of minutes even by “Quick Format” mode.  And even if the computer is powered off immediately after a destructive command has been issued (e.g. in a few minutes after the Quick Format), there is no easy way to prevent the disk from destroying the data once the power is back on.

Many commercially available data recovery tools (e.g. Intel Solid-State Drive Toolbox with Intel SSD Optimizer, OCZ SSD Toolbox) can reliably extract information from logically corrupted SSD drives. SSD drives with corrupted system areas (damaged partition tables, skewed file systems etc.) are easier to recover than healthy ones. The TRIM command is not issued over corrupted areas, because files are not properly deleted; they simply become invisible or inaccessible to the operating systems.

Modern SSDs use the SATA (Serial ATA) interface to connect to the PC and appear just like a standard hard disk to the system.

For general computer use, the 2.5-inch form factor (typically found in laptops) is the most popular. For desktop computers with 3.5-inch hard disk slots, a simple adapter plate can be used to make such a disk fit.


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