What a difference Over Provisioning makes

Introduction

Here at Myce.com the Hardware Review Team has recently been
considering how best to review and test Enterprise level Solid State Storage
solutions. As part of our deliberations, we naturally paid careful attention
to the Storage Networking Industry Association’s (SNIA) Solid State Storage
(SSS) Performance Test Specification (PTS) for Enterprise Solutions (now that’s
a mouthful J), which is widely
accepted throughout the storage industry as the professional and consistent way
to test and benchmark enterprise solutions. For further information on this,
please see SNIA SSS
PTS
.

Myce is delighted to confirm that it will be working in
partnership with Oakgate Technology Inc to implement our Enterprise Test
Bench. Please see Press
Release

It is important to note that the testing required to review
Enterprise level Solid State Storage solution is radically different to that
for consumer solutions.

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Given our track record in the review and testing of Consumer
SSDs we soon found ourselves wondering how Consumer SSDs would perform in tests
aimed at Enterprise solutions. We also noticed that Enterprise solutions
typically have a much larger amount of NAND storage dedicated for
use by their controller (the controller is the heart of an SSD and controls the
reading, writing and placement of data in the NAND storage together with the
mapping of the storage to a user’s host environment and file system). So we
also wondered if consumer SSDs would perform better in enterprise tests if we
supplemented the relatively small amount of NAND they have set aside for their
controllers by leaving space unassigned for use by the host and its file
system. In Windows this is an easy thing to do and can be achieved by leaving
space on a drive unassigned for use by partitions (by reducing the default
maximum size when creating a new volume, or by shrinking an existing partition,
in Drive Management).

The process of setting aside additional space that may
supplement the effectiveness of an SSD’s performance is commonly known as
making an additional Over Provision (‘OP’) – hence our title. An additional OP
isn’t quite the same as permanently setting aside space to be dedicated to a
controller; it does though ensure that the pool of free blocks (that do not
contain valid data) is relatively large which in turn may have the effect of allowing
a controller’s ‘Garbage Collection’ functionality to more easily stay on top of
cleaning blocks, in order to accommodate the writing of new data, especially
when faced with intense write demands. The amount of NAND permanently
dedicated to the controller is hereafter referred to as ‘normal OP’.

In the SNIA SSS PTS one of the many demanding tests
specified is known as the Write Saturation Test; this test calls for bombarding
the drive relentlessly with randomly written 4KiB blocks of random (high
entropy) data, for a protracted period of time, preferably across the full
space available for use by a host system. Part of the requirement for
enterprise testing is to record results when a drive reaches what is known as a
‘Steady State’, put simply a steady state is when the drive reaches a
performance level that does not vary significantly over time (this is important
as a drive may temporarily perform at a higher level, whilst for example, it
still has empty blocks available (for example following a secure erase, which
puts an SATA drive into a Fresh out of the Box (‘FOB’) state).

To test how Consumer SSDs perform in a Write Saturation Test
(with and without an additional OP) we used a simplified version of the test
specified in the SNIA SSS PTS. We have chosen to present the IOPS (Input/Output
Operations per Second) performance over time, in our testing.

We present our findings, which we feel you may find
surprising as well as interesting, in this article.

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Let’s move to the next page to look at the Testing
Approach…

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