Carlo and Chris, in vmwareinfo.com posted an article on iSCSI RDMs vs individually-assigned storage via the Microsoft iSCSI initiator. There were some good points for and against:
RDM Pros:
- No additional PowerPath software cost
- No guest software to install.
- No extra storage management on the guest level
- Most widely deployed solution for connecting ESX guests directly to LUN.
- Easy migration to VI4 which will most likely support PowerPath, and multiple connections per iSCSI target
RDM Cons:
- RDM LUNs must be presented to all ESX hosts in cluster to facilitate vMotion, HA and DRS.
- No load balancing for individual LUNs (ESX will establish a connection per iSCSI target, so total storage traffic can be balanced, but traffic for a single LUN cannot be balanced across multiple NICs)
iSCSI Pros:
- iSCSI guest traffic could be moved to a dedicated vSwitch, allowing for load balancing and best possible performance
- PowerPath would facilitate multipathing within the iSCSI and allow for better performance.
- Application support may be more familiar with windows guest storage connectivity support, with this configuration it would be managed the same way as physical windows hosts.
iSCSI Cons:
- Dedicated NICs for PowerPath would take away from current ESX NICS (either NICS for VM traffic, or ESX iSCSI). It could also share NICs with the ESX iSCSI vSwitch, but that could impact the performance benefits.
- Storage must be individually configured within each guest
- SRM is not supported
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The above is an almost direct copy from the site. The reason I reposted is for my own memory, and also because there needs to be additionals CONs in the iSCSI Cons section:
- You cannot use VMotion on virtual machines because the ESX host cannot confirm the presence of the boot disk on the remote server.
- VMware DRS is not supported because VMotion is required.
- You cannot perform a snapshot, suspend, or resume operation on virtual machines since this requires the current disk state to be captured, which cannot be done without a VMware Infrastructure datastore.
- You cannot clone virtual machines from a template.
- You cannot utilize the VCB framework because you are not able to quiesce virtual machines and perform a snapshot.
That’s from the SAN System Design and Deployment Guide:
This is obviously a big problem for most people. DO NOT USE third party software iSCSI initiators in virtual machines!
Cheers,
Leo