Solid State Drives

Solid State Drives (SSDs) are flash memory devices designed as direct replacements for hard disks. They are not much different from SD Cards or USB Flash Drives. However, they are designed to be mounted in regular hard drive bays and connect to regular hard drive interfaces. The exceptions are PCIe and M.2 drives that mount and connect according to their respective form factors.

Although mechanical hard drives usually offer more storage at a lower total cost, the SSD has the advantage in speed, noise, power consumption, heat, and durability. As long as mechanical hard drives have a cost-per-byte advantage, they will continue to be used alongside SSDs.

Comparing SSD performance to hard drives can be misleading. SSDs have the same advantages and disadvantages as SD Cards, USB Flash Drives, etc. As such, they have challenges with mixed reads and writes, and their capacity will degrade over time.

As an SSD ages, it tends to lose information over time when left without power. Therefore, SSDs are not recommended for archiving data.

Writing to an SSD typically takes longer than reading from it (true for any flash memory). 500 MBps read and write is the best performance for SATA SSDs. M.2 form factor drives using the NVMe (Non-Volitile Memory Express) interface may have speeds up to 7,000 MBps. Older SSDs can be very slow.

Defragmentation should be disabled on solid-state drives. Fragmentation doesn’t affect performance on SSDs as it does on spinning drives, and defragmentation causes unnecessary erase operations, decreasing the drive's lifespan. Windows disables defragmentation, Superfetch, and ReadyBoost, which are boot-time and application prefetching operations. Windows 7 and 8 also support the TRIM command, which informs the SSD when data blocks are no longer used.

A hybrid drive is a spinning disk drive with a flash memory-based cache. It gives SSD performance for frequently used data. Less frequently used data remains on the spinning disk.

Interfaces

SATA

Serial Advanced Technology Attachment (SATA) is the same interface used for internal hard drives and optical drives. SSDs of the standard form factor (see below) are direct replacements for SATA hard drives. SATA SSD performance is limited to about 550 Mbps, read or write.

PCIe

PCIe is the latest architecture for the system expansion bus.^[1] PCIe 3.0 can reach read and write speeds of 3.5 Gbps, whereas PCIE 4.0 can reach speeds of 7.5 Gbps with NVMe. PCIe 5.0 is projected to reach speeds of 10 Gbps.

NVMe

Non-Volatile Memory Express (NVMe) is a communication specification explicitly designed for today’s SSDs and the PCIe interface. PCIe 4.0 with NVMe can achieve transfer rates of 7.5 Gbps. The NVMe system resides in the SSD hardware. Windows 8.1 and later support NVMe. The Linux kernel added support in 2012, and macOS added support in 2016.

Form Factors

SSDs come in several form factors. Here are the most common form factors.

Standard

bTypical SSD as a direct replacement for a hard disk

The standard HDD form factor is a direct replacement for spinning hard drives. These are a direct fit for notebook computers with bays for spinning hard drives. They are also easily adapted to the bays for 3.5-inch hard drives in desktop computers. These SSDs connect to the standard SATA power and data connectors in computers that accept spinning hard drives.

mSATA

mSATA SSD

mSATA is one of several form factors that connect directly to the motherboard.

PCIe

PCIe, DRAM-based SSD. It uses an external power supply to make the DRAM non-volatile.

The earliest PCIe solid-state drives used dynamic RAM. These modules needed a memory controller like that on the motherboard to keep the memory refreshed. They also needed a battery backup. These may still be used on specialty systems that need the performance of DRAM-based SSDs.

An SSD that plugs into the PCIe expansion bus

Later PCIe solid-state drives used flash memory and the NVMe standard, such as the one pictured above.

SSD with Mini-PCIe interface. Some notebooks require this form factor.

Mini-PCIe looks much like mSATA but connects to the PCIe architecture instead of using the SATA interface.

M.2

M.2 (m dot two or Next Generation Form Factor) is the latest interface for SSDs. It combines PCIe, SATA3, and USB 3.0 capability into one connector.  

An M.2 PCIe x 4 NVMe 2280 SSD

M.2 SSDs come in various sizes specified by a four or five-digit number, giving the width and length in millimeters. The drive pictured above has a form factor of 2280, meaning 22 mm wide and 80 mm long, the most common size. Most M.2 SSDs are 22 mm wide.

Most new notebook computers use M.2 SSDs, and most newer desktop motherboards support one or two M.2 SSDs.The following picture shows a desktop motherboard with a single M-keyed M.2 connector. It also has two standoffs to secure the opposite end of 2280 and 22110 mm of M.2 drives.

A desktop motherboard with an M-keyed M.2 connector

M.2 SSDs and connectors are keyed to denote which standards are available.

M key

The most common is the M key, identified by five connector traces next to the key slot. The M connector provides four PCIe lanes and is compatible with NVMe and SATA.

B key

The B connector is identified by six connector traces next to the key slot. It provides two PCIe lanes and is compatible with SATA.

M + B

An M.2 SSD keyed for M and B connectors

Some SSDs are keyed to fit M or B connectors (identified by key slots in the M and B positions). Such drives are typically two-lane PCIe or SATA drives. They can be connected to M or B connectors but cannot use the extra PCIe lanes or NVMe.

In addition to the above standard form factors, many adaptations exist for various flash media to be used as SSDs. The most common adapt SD cards to standard SATA connectors.