Hi all,
I am trying to get my head around the VSA and how it integrates with a hosts local storage. I have read all the documentation I can find, but feel like i haven't really been given the level of detail I need. I'm hoping somebody else has already tried this functionality, can see where I'm confused, and help me.
Here's where and why I am confused:
"VSA can be deployed in a two-node or three-node configuration. Collectively, the two or three nodes in the VSA
implementation are known as a VSA storage cluster. Each VMware ESXi server has a VSA instance deployed to it
as a virtual machine. The VSA instance will then use the available space on the local disk(s) of the VMware ESXi
servers to present one mirrored NFS volume per VMware ESXi. The NFS volume is presented to all VMware ESXi
servers in the datacenter."
Source: vSphere® Storage Appliance TECHNICAL WHITEPAPER
What is "the available space"? Does this mean I have dedicated, unformatted disks (with or without RAID?), or is it creating a virtual software RAID set from spare VMFS space on an existing set (the set I used to install ESXi on, for example)?
Also,
"• Synchronous mirroring of data stores, with RAID 1 across
nodes, and RAID 10 within each node"
Source: VMware vSphere Storage Appliance - Product Datasheet
Huh? Where's RAID 10 coming from? Is this saying that I should dedicate a RAID 10 set for the VSA's storage? If so, where does the local replica sit? If it's on the same storage then I'm only getting 25% of my disk space usable....
For example, if I have 4 1TB drives (~4TB) and put them in a RAID 10 set, then I have only 2TB usable. If the VSA then uses 1/2 this for usable storage and 1/2 for the RAID 1 it creates across nodes (see image below, from the data sheet), then that leaves me only 1TB usable from my 4TB of storage.
Is this right? RAID 10 is expensive enough as it is, without losing 50% twice!
Finally, I've read various internet posts which state that you cannot use external storage (e.g. HP MSA/Dell MD's etc.), but can't see why the VSA would care. Unless they've gone out of their way to forbid the use of these, the resulting drives would appear local to the host anyway (the onboard RAID controller would RAID the external shelf and present what appears to be a local drive). Can anybody confirm or deny this for definate?
Thanks all - hopefully this post will help other, equally confused individuals like myself. Hopefully I'm not the only one! 
I believe you're correct on the 25% usable space math. I had a discussion about this with a storage engineer at the Chicago VMUG and we came to the same conclusion.
Finally, I've read various internet posts which state that you cannot use external storage (e.g. HP MSA/Dell MD's etc.), but can't see why the VSA would care. Unless they've gone out of their way to forbid the use of these, the resulting drives would appear local to the host anyway (the onboard RAID controller would RAID the external shelf and present what appears to be a local drive). Can anybody confirm or deny this for definate?
I'd imagine that if you had an MSA you'd just hook the 2 (or 3) servers directly into it and use it as shared block storage over the SCSI cables. I have an MSA50 in my work lab shared among 2 hosts. Certainly beats the 25% penalty.
I haven't tried setting up a VSA using the MSA so I can't testify either way.
