EMC Symmetrix DMX-3 User Manual

Global memory performance features
121
Performance and Optimization
PermaCache option
The PermaCache option allows you to permanently assign
mission-critical data requiring very high performance to global
memory. A variable number of contiguous cylinders on the disk
drives can be reserved for PermaCache backup.
PermaCache is best used for infrequently accessed data that needs
instantaneous response because this data is normally not in global
memory when it is requested. The Symmetrix large global memory
and intelligent global memory algorithms try to keep frequently
accessed data in global memory, making its assignment to
PermaCache unnecessary.
Other Symmetrix options that require global memory resources affect
the amount of global memory available for PermaCache. For
example, enabling the RAID 6 options (in (6+2) or (14+2) mode) may
require adjusting memory assignments by reducing the existing
PermaCache area, lowering the write-pending ceiling of the system,
or reducing the number of Symmetrix RAID 6 groups defined.
Global memory
requirements
Your system must be configured with more than the minimum (base)
amount of global memory for it to use part of that memory as
PermaCache. To determine the minimum memory requirement for
your Symmetrix configuration, consult your local EMC Sales
Representative.
Power failures If a power failure occurs, records that have been updated in
PermaCache will be destaged to disk.
Global memory
director replacement
If a global memory director requires online replacement, PermaCache
is unaffected, provided that the replacement board contains the same
amount or greater of global memory. Replacement with a smaller
capacity memory requires that the system be brought offline and
reinitialized before PermaCache will function.
Online PermaCache
configuration
changes
Symmetrix Enginuity permits PermaCache configurations to be
changed while the Symmetrix system is online to the host. Enabling
PermaCache modifications to take place through an online
configuration change allows users to set a configuration with no
disruptions and avoids requirements for future maintenance.

EMC Symmetrix DMX-3 Specifications

General

Brand	EMC
Model	Symmetrix DMX-3
Category	Storage
Language	English