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» VIEW ALL POSTS Mar 24 2008   8:03AM GMT

What to do with internal disk space on VMs



Posted by: Joe Foran
Tags:
Joseph Foran
Storage
Virtualization management
VMware

If one thing annoys me to no end, it is unused capacity.

That’s why I like virtualization. And it’s also why I like grid computing. Heck, that’s why I like cheeseburgers (there’s no empty space in my stomach after a trip to In-and-Out.) Virtualization makes efficient use of existing hardware to control costs. VMware environments often have host servers with more than a small RAID1 array in them because they were existing servers retasked as ESX hosts. Sometimes this space gets used for ISO file storage, sometimes it gets used for virtual machine toys (so called gray- or black- boxes) and sometimes it’s used for production VMs. Labs are often set up on the local storage with machines that have no production value hosted, which makes great use of that space (but perhaps not as good of use of CPU or memory for production machines).

A case study in space, storage and VMs
Then there are times when I walk into sites like the one I saw last week. They had a virtual-machine iSCSI SAN set up on each ESX host homed to the local storage. This was in addition to their FC SAN, by the way. They even ran part of their production environment off of it, using the unused internal disk space on the ESX servers to store virtual appliances that ran iSCSI Targets in them, similar to what I described in an earlier post. What a great use of space. Kudos to them!

I was concerned about how they were to keep those virtual machines up in the event of host failure. First I was told that they put in a poor-man’s round robin, which is to say, host 1 has iSCSI SAN 1, host 2 has iSCSI SAN 2, 3 had 3, and 4 had 4, and they all replicated to each other; VMs hosted by ESX 1 were on SAN 2, those on ESX 2 were on SAN 3, those on ESX 3 were on SAN 4, and those on ESX4 were on SAN1. Then, anti-affinity rules were used to prevent VMotioned VMs from winding up on the same host as the SAN on which their files existed. The replication prevented any single SAN failure from becomming a nightmare. They hadn’t done anything unusual on the network side though, which bothered me (I would prefer dedicated physical NICs for the SAN VMs!), but their performance testing showed no need for the extra NICs to be added. It’s a little hard to follow-the-leader, but it worked reasonable well. This was done with a variety of open-source packages, some of which I had never seen before. I recognized IET right away, but the SAN appliances were all custom builds, and it took me some time to figure out what was what and what was going on where. It was not an efficient use of that internal disk space because of all the replication across servers being, essentially, mirrors of mirrors.

Ingenious? Yep. Complicated to track? Yep. Functional. Yep. Needing of a little less work to manage? Yep.

There are commercial products to do this, one of them is LeftHand Network’s Virtual Storage Appliance. I’ll post more about my experiences with this very soon.

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