Solid-state Drive - Comparison of SSD With Hard Disk Drives

Comparison of SSD With Hard Disk Drives

See also: Disk drive performance characteristics

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.

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).	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 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. 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 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 (Filesystem specific)	There is limited benefit to reading data sequentially (beyond typical FS block sizes, say 4KB), making fragmentation negligible 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.
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. However the drives firmware may exchange damaged blocks with spare areas so bits and pieces may still be present.
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.