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26 DS6000 Series: Concepts and Architecture
the data is written to volatile memory on one controller and persistent memory on the other
controller. The DS6800 then reports to the host that the write is complete before it has
actually been written to disk. This provides much faster write performance. Persistent memory
is also called NVS or non-volatile storage.
Figure 2-4 DS6000 architecture
When a host performs a read I/O, the controllers fetch the data from the disk arrays via the
high performance switched disk architecture. The data is then cached in volatile memory in
case it is required again. The controllers attempt to anticipate future reads by an algorithm
known as SARC (sequential prefetching in adaptive replacement cache
). Data is held in
cache as long as possible using this smart algorithm. If a cache hit occurs where requested
data is already in cache, then the host does not have to wait for it to be read from the disks.
Controller
card 0
SAN fabric
Controller
card 1
Power PC
chipset
Power PC
chipset
Persistent memory
Volatile
memory
Persistent memory
Volatile
memory
host adapter
chipset
host adapter
chipset
20 port fibre channel switch
20 port fibre channel switch
20 port fibre channel switch
20 port fibre channel switch
device adapter
chipset
device adapter
chipset
Server
enclosure
First expansion
enclosure (if present)