EMC CX500I Configuration Guide - Page 122

Storage-System Hardware Storage-System Caching 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. SP Management Network Connection 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. SP iSCSI Network Connection 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. 6-6 EMC CLARiiON CX300, CX500, CX500i, and CX700 Storage Systems Configuration Planning Guide