iSCSI or VMDK?

[BennyZhang]

Typically, when building a file server, we create an iSCSI connection to our SAN for the files, only leaving OS files on the VMDK. This, however, creates a problem for backup using Veeam as Veeam can't backup iSCSI targets within a guest OS.

Should we create the entire VMware guest using VMDK? What's the pros and cons?

[Gostev]

The only tangible benefit of in-guest iSCSI approach is no 2TB limit. And multiple drawbacks due to loss of all storage virtualization benefits.
VMDK has no cons aside of 2TB limit, but brings all the benefits of storage virtualization, and is definitely the recommended way to deploy VMs.

[BennyZhang]

Thanks, Gostev. Can you elaborate a little on the VMDK benefits?

I assume, for example, for the virtual file server, you recommend that I create 2 virtual hard drives like this:
1. OS drive is on Vmware Datastore 1 on SAN1
2. File drive is on Vmware Datastore 2 on SAN2

[dellock6]

Hi,
with VMDK you can snapshots them so you can do backups (you can also do it using vRDM, not on pRDM), you can use storage vmotion (and Storage DRS) or cold migration. You can monitor their size directly inside vCenter, and there no real performance benefit in RDM vs VMDK.
If you need larger disks, in my opinion is better to avoid both 2Tb vmdk and RDM. If you need to move them around it's a pain, better using multiple medium-sized vmdk and join them via Guest OS (like LVM or others).

[BennyZhang]

I'd assume for all virtual servers such as Exchange, you recommend to use VMDK for all drives right?

[dellock6]

I'd recommend using VMDK for every disk in a VMware environment unless you really need RDM, so basically yes, also for Exchange.

[Gostev]

The top benefit of using VMDK is storage virtualization, which provides for VM portability. All other benefit are just "consequences" basically. Just as Luca noted, not having your storage virtualized kills most of the great features and possibilities that virtualization brings, especially around data protection and high availability.