Symmetrix DMX-3 operation
103
Symmetrix DMX-3 Input/Output Operations
Symmetrix global
memory
management
The Symmetrix DMX-3 uses the same memory principle as host
systems, but with enhanced caching techniques in global memory.
Figure 20 on page 103 illustrates global memory use in Symmetrix
systems.
Figure 20 Symmetrix global memory management and data flow
In Symmetrix systems, the channel directors and disk directors share
global memory. Symmetrix channel directors attach to the CPU
channels as well as to global memory. Symmetrix disk directors
attach to global memory as well as the disk drives. The Symmetrix
directors perform the following functions:
â—† The channel director handles I/O requests from the host. It
accesses the directory in global memory Figure 20 on page 103 to
determine if the request can be satisfied within global memory.
The directory contains information on each memory page and
blocks within each page.
â—† Tag-based Caching (TBC), a Symmetrix Enginuity Least Recently
Used (LRU) algorithm divides global memory into groups of
several hundred slots called TBC groups. In the TBC data
structure, four bytes represent each slot. The four bytes contain
information about the last time the system most recently accessed
this slot, whether the slot is write pending, and other slot
attributes. The bytes that represent the slots of a TBC group are
contiguous in global memory.
The TBC LRU algorithm determines which data residing in global
memory has the lowest probability that the system will access it
soon, and discards this data to make room for new data the
system is about to access.
Host
system
Disk
Channel
director
Directory
Global
memory
Disk
director
Symmetrix
SYM-000450
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