Solid state drive technology for ProLiant servers

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Introduction
HP is now delivering solid state storage devices based on flash memory in addition to traditional disk
drives based on spinning magnetic media. Solid state drives (SSDs) are the most familiar of these new
devices, since they are plug-compatible with disk drives and used with traditional SATA/SAS disk
controllers. But we are also introducing new types of flash-based storage devices for ProLiant servers
that will significantly improve performance for certain types of applications. This technology brief
provides an overview of solid state storage and the new high performance products that we at HP are
developing using this new technology.
Flash memory technology
Most solid state drives use flash memory technology, a non-volatile computer memory that can be
electrically erased and reprogrammed. There are two configurations, NOR flash and NAND flash.
NOR and NAND flash both store information in arrays of floating-gate transistors called “cells.” But
they differ in how the cell arrays are organized and accessed. NOR flash memory cells connect in
parallel to the bit lines, letting you read and program the cells individually. NAND flash memory cells
connect in a series, and you can only read or program the cells as a group.
NAND’s architecture allows you to create memory arrays with almost twice the density of comparable
NOR memory and at a lower cost. As a result, most devices use NAND flash memory.
Single-level and multi-level cell NAND flash
There are two primary types of NAND flash technology:
 Single-level cell (SLC) technology works by storing a single level of charge in each cell, representing
a single bit of information.
 Multi-level cell (MLC) technology stores one of four different charge states in a cell. This allows each
cell to represent 2 bits of information, effectively doubling storage density.
NAND flash memory using multi-level cell technology has quickly become the primary flash
technology in consumer products. Compared to SLC, MLC technology has several characteristics that
make it less desirable for creating the type of higher performance, high reliability devices required for
server storage (Table 1), including the following:
 Higher internal error rates caused by the smaller margins separating the cell states, necessitating
larger ECC memories to correct them
 Shorter lifespan in terms of maximum number of program/erase cycles
 Slower read performance and significantly slower write (program) performance
Table 1. Primary characteristics of SLC and MLC flash
SLC flash MLC flash
Random access 25 microseconds 60 microseconds
Serial access 50 nanoseconds 30 nanoseconds
Page program (write) 200 microseconds 800 microseconds
Maximum program/erase cycles 100,000 @ 1 bit ECC 5000 – 10,000 @ 4 bit ECC
As Table 1 shows, MLC NAND flash has comparatively poor read and write performance. More
important, SLC flash has a program/erase lifecycle―often referred to as endurance―that is 10 to 20