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Best Option for Hyper-V Disks


RDBenfield
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Some of you may have seen my other post about building a new server to host VMs. As I wait for the money to build out the server, I'm left with some time on my hands to contemplate how I will build out the environment (not just physically, but virtually). One of the things I've been trying to decide, is how to setup the disks for the VMs. There are a couple of questions I have around this. But the main thing I want to know is what type of disks you use and why? The way I see it I have the following options.

  1. Direct Attached Storage aka Passthrough
    • I have not used these, but I would assume the main advantage is performance and the main disadvantage is inflexibility?

[*]Fixed

  • I have used these and from what I understand, they should offer the best balance of performance while still maintaining some flexibility, especially over time?

[*]Dynamically Expanding

  • I have also used these and the main advantage of these would seem to be the fact that you can over provision? So if you have a 500GB drive, you could create more than 500GB in dynamically expanding virtual disks, as long as they didn't consume more than the 500GB in actual stored files.

[*]Differencing

  • I have used these as well. Ok, so this one seems the most promising and the most problematic, all at the same time. I love the idea of differencing disks. But in the long term, it seems like it would be more trouble than it is worth. But if you do go with this format, what is your strategy? See more below.

So using differencing disks would seem to be all about the capacity benefits (especially if you will have a bunch of VMs all using the same OS). But in all honesty, it could also be about performance benefits... Consider the following:

 

You could have one (or few) fast small SSDs and some big cheap storage. Then create a parent disk for each OS on the SSDs and create all of your differencing disks on your large cheap storage. Then you could keep the bulk of the files all sharing the same instance on the fast SSDs. The problem would seem to be that over time (and patching, and application installs, etc) the files in the parent disk will be referenced less and less and the files in the child (differencing) disk will increase in number and therefore size and inefficiency. The more this occurs the less your capacity and performance efficiency. Right?

 

At the end of the day, I would love to see something a little more dynamic. Something that allows you to create completely virtualized disks, partitions and volumes, where the actual file storage is not tied to those containers. I would love to see some kind of single instance storage, where you get the benefits of a parent disk for differending, but more dynamic. So that files can be updated, but if a file is updated across all differencing disks based off a parent disk, perhaps it could automatically be merged into the parent disk and thus only have to be stored once on the fast storage. So if you patch all of the VMs based on the same parent disk. you dont end up with multiple copies of the updated files all stored on slow disks. But I realize there are a lot of things going on in the background that make this seemingly simple concept, very complex to implement. I'm also sure you would take a performance hit for this, but as long as the hit wasn't too great, I think it would be worthwhile. In any case, for now, we have what we have and we have to work with that.

 

So what is the best option? Only use differencing disks for more stable VMs like your infrastucture servers? Don't use them at all? Use them for everything, just create a parent disk for each OS? Also, if you you do use differencing disks, do you put the parents on your fast storage and differencing disks on mass storage? Or do you put everything on the SSDs, or everything on the mechanical? Perhaps put all the parent disks on the SSDs and then put some differencing disks on SSD and others on HDDs based on use? Whether you use Differencing disks, Dynamically Expanding disks or Static/Fixed disks, do you separate your page files on to your fast storage or your slow storage or leave them in the main virtual disks or what?

 

I'd really like to hear any experiences you have had on this subject.

 

Thanks,

Richard

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Guest no-control

I use all of these depending on my application. Direct I use for storage So I have a RAID5 storage array that I passthrough to my WHS2011 VM and that is where my data sits.

I use fixed for VMs where I don't monkey around with them. I have reference VMs for each OS that I support. Each of those is only a few GB more than the OS install requires for patching. I also use fixed for my linux security OS installs.

Dynamic is what I use for almost all of my VM's, Using only actual space required but not limiting me to a fixed size. This allows me to install applications on client VMs and Server packages to server platforms.

I've only used differencing for testing and for the most part do not really care for it. Dynamic works for me in 90% of applications and do not see any reason to move to differencing.

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Thanks for the feedback no-control. Can you direct attach storage to multiple VMs at the sametime? I guess even if you could, maybe it would be best to still only attach it to one server and share it out from that one?

 

Regards,

Richard

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Guest no-control

You can direct attach multiple disks to multiple VMs but you cannot share the resource across multiple VMs. Direct attach works just like the real world. You cannot physically share a drive between to computers.

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Ok, that makes sense... Thanks again for all of the guidance... I'm getting really excited about this project! I'm hoping to document it on YouTube and with some articles. But we'll see.

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