IBM N Series, Hardware Manual

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IBM System Storage N series Hardware Guide
12.1 About Flash Cache
Flash Cache (previously called PAM II) is a set of solutions that combine software and
hardware within IBM N series storage controllers. It increases system performance without
increasing the disk drive count. Flash Cache is implemented as software features in Data
ONTAP and PCIe-based modules with either 256 GB, 512 GB, or 1 TB of flash memory per
module. The modules are controlled by custom-coded Field Programmable Gate Array
processors. Multiple modules can be combined in a single system and are presented as a
single unit. This technology allows submillisecond access to data that previously was served
from disk at averages of 10 milliseconds or more.
12.2 Flash Cache module
The Flash Cache option offers a way to optimize the performance of an N series storage
system by improving throughput and latency. It also reduces the number of disk spindles and
shelves that are required, and the power, cooling, and rack space requirements.
A Flash Cache module provides another 256 GB, 512 GB, or 1 TB (PAM II) of extended cache
for your IBM System Storage N series storage subsystem. The amount depends on the
model. Up to eight modules can be installed. Each module must be installed on a PCI express
slot, and uses only another 18 W of power per module. Extra rack space and ventilation are
not required, which makes it an environmentally friendly option. Figure 12-1 shows the Flash
Cache module.
Figure 12-1 Flash Cache Module
12.3 How Flash Cache works
Flash Cache replaces disk reads with access to an extended cache that is contained in one or
more hardware modules. Your workload is accelerated in direct proportion to the disk reads
replaced. The remainder of this document focuses on different workloads and how they are
accelerated. It also describes how to choose and configure the best mode of operation and
how to observe Flash Cache at work.
Tip: This solution is suitable for all types of workloads, but provides the greatest benefit
from IBM System Storage N series storage subsystems that serve intensive random read
transactions.