The Microsoft Windows implementation of iSCSI includes the capability to enable Header and/or Data Digests. These protocols provide an extra measure of data integrity to the standard TCP checksums that are present in the iSCSI traffic flow. A Header Digest will perform a checksum of the iSCSI Protocol Data Units (PDUs) at the 48-byte header of the iSCSI TCP packet. A Data Digest will perform a checksum against each data segment included in the iSCSI TCP packet that is attached to the PDU.
Enabling either or both of these Digests does require additional CPU usage on the initiator as well as the target to calculate and append checksums. This process occurs bi-directionally (incoming and outgoing) on the initiator and the target. Every storage environment is unique and when altering settings such as Digests, it must first be fully tested to accurately assess overall impacts on IOPS and CPU performance on the iSCSI network and participating devices. In our lab testing, we found slight to moderate increases in CPU overhead based on load when these options were enabled, with a minor effect on IOPS. The overall recommendation from Pure is to keep these options disabled if they are not required in your environment. This recommendation should not be considered a "best practice" configuration as the decision to enable these options are based on the specific environments and workloads and would need to be tested.