Understanding endurance and performance characteristics of HP solid state drives

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Introduction
As more organizations seek to harness the power of information, the demand for data intensive and
transactional workloads such as data warehousing, real-time analytics and virtualized environments is
expanding. For that reason, solid state storage technology is becoming more mainstream because it
delivers the performance, energy efficiency and high density ideal for these applications. HP solid state
drives (SSDs) for ProLiant servers offer significant performance benefits over traditional disk drives for
applications requiring high random I/Os per second (IOPs) performance. In addition, HP ProLiant
Gen8 servers and drive controllers have been designed to optimize solid state media performance,
delivering up to 6 times the performance with SSDs versus previous generations.
Because they are plug-compatible with traditional SATA and SAS drives, we tend to think of SSDs in the
same terms as traditional disk drives. But SSDs have unique functional characteristics that require us to
re-think some of our usual assumptions when using them in server-based IT environments. This paper
provides an overview of two of the unique aspects of SSDs—SSD endurance and SSD performance
characteristics in server applications.
SSD endurance
SSDs are compatible with the SAS and SATA interfaces originally defined for reading and writing data
to hard disk drives (HDDs). But behind these interfaces, an SSD is a completely different animal. Instead
of storing data as magnetic fields on spinning disks, SSDs store data in NAND memory cells. This
essential difference profoundly influences both the endurance and data retention characteristics of SSDs
when compared to traditional disk drives.
An introduction to endurance
In terms of data storage, endurance refers to the durability of the medium on which the data is stored.
How long will the medium last before it wears out and can no longer effectively store data? With disk
drives, endurance is rarely an issue. The effective lifespan of the magnetic medium of the disks is
typically longer than the time that most disk drives are in service. With SSDs, this is not true. To
understand SSD endurance, we first need to review some basics of SSD architecture.
NAND organization
NAND flash memory arrays consist of pages and blocks. Pages are the smallest units of NAND
memory that you can address and write to. Page size can vary between different NAND
implementations, but they are typically 4 KB or 8 KB. Once you write to a page, you cannot simply
overwrite it the same way you could a disk sector. You must first erase its contents. Pages are
organized into NAND blocks, which are typically 256 KB to 1 MB in size and should not be confused
with the 512 byte logical block of the SATA or SAS interface.
There are two important things to know about NAND blocks. The first is that you can only erase NAND
memory at the block level, not at the page level. This means that the SSD controller must relocate and
remap any valid data in a block before the controller can erase and write new data to it. The second
point is that the lifespan of NAND blocks is limited. They can only be erased and re-written a certain
number of times. This is the basic reason for the limited endurance of SSDs.
SLC versus MLC NAND
There are two primary types of NAND memory—single level cell (SLC) and multi-level cell (MLC). SLC
NAND stores a single bit in each memory cell, and MLC NAND stores 2 or more bits per cell. MLC is