EMC CX700 Planning Manual page 122

Storage-System Hardware
Storage-System Caching
SP Management Network Connection
SP iSCSI Network Connection
6-6 EMC CLARiiON CX300, CX500, CX500i, and CX700 Storage Systems Configuration Planning Guide
Storage-system caching improves read and write performance for
several types of RAID Groups. Write caching, particularly, helps
write performance — an inherent problem for RAID types that
require writing to multiple disks. Read and write caching improve
performance in two ways:
For a read request — If a read request seeks information that's
already in the read or write cache, the storage system can deliver
it immediately, much faster than a disk access can.
For a write request — the storage system writes updated
information to SP write-cache memory instead of to disk,
allowing the server to continue as if the write had actually
completed. The write to disk from cache occurs later, at the most
expedient time. If the request modifies information that's in the
cache waiting to be written to disk, the storage system updates
the information in the cache before writing it to disk; this requires
just one disk access instead of two.
A standby power supply (SPS) protects data in the cache from power
loss. If line power fails, the SPS provides power to let the storage
system write cache contents to the vault disks. The vault disks are
standard disk modules that store user data but have space reserved
outside operating system control. When power returns, the storage
system reads the cache information from the vault disks, and then
writes it to the file systems on the disks. This design ensures that all
write-cached information reaches its destination.
During normal operation, no I/O occurs with the vault; therefore, a
disk's role as a vault disk has no effect on its performance.
Each SP has an Ethernet connection through which the EMC
Navisphere® management software lets you configure and
reconfigure the LUNs and Storage Groups in the storage system.
Each SP connects to a network; this lets you reconfigure your system,
if needed, should one SP fail.
Each SP in an iSCSI storage system has iSCSI Gigabit Ethernet
connections through which the target and initiator communicate.
Each iSCSI port connects to a Gigabit Ethernet port on a network
device such as a switch or router, or directly to an initiator iSCSI port
on an HBA or NIC.