vSphere Storage - ESXi 5.1 - Documentation - VMware

Discovery
A discovery session is part of the iSCSI protocol, and it returns the set of targets you can access on an iSCSI
storage system. The two types of discovery available on ESXi are dynamic and static. Dynamic discovery
obtains a list of accessible targets from the iSCSI storage system, while static discovery can only try to access
one particular target by target name and address.
For more information, see “Configuring Discovery Addresses for iSCSI Adapters,” on page 99.
Authentication
iSCSI storage systems authenticate an initiator by a name and key pair. ESXi supports the CHAP protocol,
which VMware recommends for your SAN implementation. To use CHAP authentication, the ESXi host and
the iSCSI storage system must have CHAP enabled and have common credentials.
For information on enabling CHAP, see “Configuring CHAP Parameters for iSCSI Adapters,” on page 102.
Access Control
Access control is a policy set up on the iSCSI storage system. Most implementations support one or more of
three types of access control:
n By initiator name
n By IP address
n By the CHAP protocol
Only initiators that meet all rules can access the iSCSI volume.
Using only CHAP for access control can slow down rescans because the ESXi host can discover all targets, but
then fails at the authentication step. iSCSI rescans work faster if the host discovers only the targets it can
authenticate.
Error Correction
Chapter 10 Using ESXi with iSCSI SAN
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.
NOTE 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,” on
page 108.
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