To protect the integrity of iSCSI headers and data, the iSCSI protocol defines error correction methods known as header digests and data digests.
Both parameters are disabled by default, but you can enable them. These digests pertain to, respectively, the header and SCSI data being transferred between iSCSI initiators and targets, in both directions.
Header and data digests check the end-to-end, noncryptographic data integrity beyond the integrity checks that other networking layers provide, such as TCP and Ethernet. They check the entire communication path, including all elements that can change the network-level traffic, such as routers, switches, and proxies.
The existence and type of the digests are negotiated when an iSCSI connection is established. When the initiator and target agree on a digest configuration, this digest must be used for all traffic between them.
Enabling header and data digests does require additional processing for both the initiator and the target and can affect throughput and CPU use performance.
Systems that use Intel Nehalem processors offload the iSCSI digest calculations, thus reducing the impact on performance.
For information on enabling header and data digests, see Configuring Advanced Parameters for iSCSI.