VMware ESX vs ESXi
Both ESX and ESXi hypervisors use the same vmkernel, so the underlying core infrastructure is the same. Although there are differences in how ESXi is managed from ESX. The differences between ESX and ESXi are narrowing with the new release of vSphere 4.1, putting both hypervisors at an equivalent level to each other.
With ESX the virtualization kernel (vmkernel) works in conjunction with the service console. The service console provides a management interface for ESX. This architecture allows administrators to log into the Service Console to run various scripts and commands for configuration and diagnostics.
ESXi can be installed to disk, or as the Embedded Edition typically sold by hardware vendors. ESXi allows for both disk and disk-less Host servers, providing increased flexibility and lower hardware and power/cooling costs when using diskless servers. There are two variations of ESXi, VMware ESXi Installable and VMware ESXi Embedded Edition. ESXi can be upgraded to a commercial version.
In future releases of vSphere VMware will only provide ESXi. Existing ESX hosts will need to be migrated to ESXi to stay current with the latest hypervisor code base. The differences between ESX and ESXi are becoming so minor that it’s more an issue of getting used to managing the hypervisor slightly differently than in the past. Just as the Windows operating system has evolved, so is VMware’s hypervisor evolving.
Upgrades to newer versions of ESXi have an advantage over using ESX, since a single image file is used for the ESXi upgrade. There are no issues with patches or the order or dependency of patches with ESXi. Reverting back to a previous ESXi version is much easier than with ESX since old versions are automatically saved in a backup partion.
Regardless of whether you use ESX or ESXi, using a SAN (Storage Area Network) will let you take advantage of the full range of VMware’s advanced features such as vMotion, HA, DRS and DPM. The decision whether to use a FC (Fiber Channel) or an iSCSI SAN is becoming easier to make now that iSCSI performance can be proven to be more than enough for most SMB customers, and in some cases can even be faster than FC. There are many factors that affect SAN performance from the storage controllers in the SAN to the number and speed of the disks to the actual SAN fabric and Adapters in the servers.