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Demartek SSD Primer

Updated 19 April 2012

By , Demartek President

Because of the great interest in solid state storage (SSS) technology, we have compiled this summary document providing some basic information for solid state drive (SSD) or solid state disk technology. The terms SSS and SSD are often used interchangeably. Technically, SSD refers to a specific form factor, while SSS refers to any solid state storage form factor.

Solid state storage devices are computer storage devices that use memory technology for the storage media rather than traditional magnetic media such as hard disk drives (HDD) or tape drives. These SSS devices can be made with DRAM memory technology or Flash memory technology, or sometimes both. These devices appear to the host operating system as storage devices.

This document will be updated periodically and may become larger over time. Contact us if you’d like to see additional information in this document.

The Demartek SSD Deployment Guide is now available. This guide covers NAND flash characteristics such as SLC, MLC, endurance and performance. It also discusses SSD form factors and data placement strategies such as caching and tiering. A similar deployment guide that we have already produced is the Demartek iSCSI Deployment Guide.


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Contents


Acronyms

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Form Factors

SSS technology is available in several form factors. These can be:

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NAND Flash Basics

NAND flash is a type of flash memory that is similar in basic function to Electrically Erasable Programmable Read-Only Memory (EEPROM). The underlying technology is a floating-gate transistor.

Data in NAND flash is erased and programmed (written) in blocks. This process is known as the program-erase cycle. NAND flash blocks are typically 4KB in size, though some are larger. NAND flash is low-power, low-heat, low-weight and low-noise when compared to hard disk drives.

NAND flash is available in two basic types: Single-level Cell (SLC) and Multi-level cell (MLC). SLC flash has one bit per cell, and is generally designed for enterprise applications. MLC flash has more than one bit per cell, and is generally designed for consumer applications. There is a relatively new category known as eMLC (Enterprise MLC) that is MLC flash but with some of the characteristics of SLC flash. The enterprise characteristics of eMLC flash are provided primarily by the intelligence within the low-level flash controllers on the device.

MLC flash is available as two bits per cell, three bits per cell or four bits per cell. These are generally known as MLC-2, MLC-3, and MLC-4. Three bits per cell is also sometimes known as Triple-level Cell (TLC). As more bits per cell are added, capacity increases while performance and endurance decreases. The chart below provides a sliding scale of various factors for each of the types of flash.

NAND Flash Characteristics by Bits per Cell

SLC MLC-2 MLC-3
(TLC)
MLC-4
Bits per Cell 1 2 3 4
Performance Fastest     Slowest
Endurance Longest     Shortest
Capacity Smallest     Largest
Error Probability Lowest     Highest
Price per GB Highest     Lowest
Applications Enterprise Mostly Consumer Consumer Consumer

SLC flash typically has 10x - 20x better endurance than MLC-2 flash. SLC has a typical life of at least 100,000 write (program-erase) cycles per bit. MLC-2 flash has better endurance than MLC-3 or MLC-4. MLC-2 flash has a typical life of 3000-10000 write cycles per bit. MLC-3 flash has a typical life of 300-3000 write cycles per bit. Enterprise MLC (eMLC) generally uses MLC-2 flash and has advanced features in the flash controllers that are able to typically provide 20,000-30,000 write cycles per bit.

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NAND Flash Endurance

There has been enough concern about NAND flash endurance, especially for enterprise applications, that the memory standards body, JEDEC, has defined standards for NAND flash endurance. They have divided NAND flash devices into the following two categories: These standards, JESD218A and JESD219, are available on the JEDEC web site.

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NAND Flash Performance

The storage standards body, SNIA, has created performance testing standards for SSS technology. SNIA has defined performance testing standards for two classes of NAND flash devices: According to the SNIA Solid State Storage (SSS) Performance Test Specification (PTS), a typical SSS device, taken Fresh Out of the Box (FOB), and exposed to a workload, experiences a brief period of elevated performance, followed by a transition to Steady State performance. The SSS PTS ensures that performance measurements are taken in the Steady State region, representing the device’s performance during its normal working life. The test specifications for client and enterprise devices are available on the SNIA web site.

Demartek conducts performance testing for SSD devices and uses the SNIA SSS PTS or a subset of it, depending on the specific type of testing required.

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Cost Metrics

Many people are familiar with cost per gigabyte ($/GB) for storage pricing, and have attempted to compare SSD pricing and HDD pricing on this basis. While this is one reasonable measure, there are other metrics that are appropriate for SSS technology. The following table provides general ranges of these other metrics for the device types as a whole. Individual devices will have specific metrics that generally fall within the ranges described below. The metrics for eMLC devices tend to between the SLC and MLC metrics shown below, with some overlap.

Cost Metrics Comparison of SSD and HDD Technologies

$/GB $/IOPS IOPS/Watt
SSD (SLC) $10-$40 $0.005-$0.15 1000-15000
SSD (MLC) $1-$3 $0.004-$0.05 1000-15000
HDD (Enterprise) $0.50-$1.00 $1-$3 10-30
HDD (Desktop) $0.05-$0.10 $1-$4 10-40

Our conclusion based on this data:

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Data Placement: Caching vs. Primary Storage

A key factor for SSD technology is deciding how to place data on these devices. SSD technology can be deployed for data caching, primary data storage, or both. Solutions for both types of data placement deployments are available from individual devices in a server up to large-scale, datacenter-class storage arrays. We have conducted some performance tests using these two data placement techniques. These evaluation reports are available in the Demartek SSD Zone. Here is an overview of these two data placement strategies and techniques.

Caching

Primary Storage

Some vendors who provide only one of these solutions will probably provide the other type of solution in the future. Some vendors allow SSDs to be split into two parts, one dedicated to caching and the other dedicated to primary storage possibly with their automated storage tiering software.

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