Multiple virtual discs

[djones]

Is it possible, sensible (or what is 'best' practice) to connect two virtual discs to one virtual machine in Hyper-V? I was just about to begin setting up and began thinking about OS/data separation. How should this idealing work/be setup?

[tmcd35]

Depends on what task the VM is being given. Certainly is possible and for things like File Servers this is exactly how I'd set it up - seperate VHD for OS and Data. That said most of my servers are single task so they just have a single VHD with the software loaded for that task. I tend to set the HDD size to the maximum supported, 127Gb I think, and use auto-expanding drives. Most of my images are around the 10-20Gb mark but can grow to the full 127Gb if ever needed thus doing away with the need for seperate hard drive images in most cases.

[djones]

This will be a file server. I was thinking about a fixed 50Gb VHD for the OS and another dynamic VHD for the DATA. Any thoughts?

[danrhodes]

If the disk is going to be used for storing alot of files (FileServer) then it may be better to connect a physical drive using Raw Device Mappings to a SAN rather than using a VHD.

 

D

[tmcd35]

Personally I'd say never used fixed disks without good reason. It'd come back and bite you eventually. But yes, I'd go for two dynamic drives, 1 for the OS and 1 for the file server DATA.

[djones]

Just don't have (and won't have) a SAN. Would it be better to keep this as a physical box then rather than a VM?

[danrhodes]

What virtualisation do you currenlty have in place? Do you plant to consolodate all your H/W onto Virtual Machines as if your only doing this 1 then it may be a fruitelss effort.

 

If you already have virtualisation in place and don't currently have a SAN, or have any plans to purchase a SAN then Dynamic Disks are the way, as said above, it will come back and haunt you one day if you go with fixed disks.

 

D

Edited May 24, 2010 by danrhodes

[djones]

Yes. Only have one physical (2003 R2 - does everything) box at the moment and have just purchased another server. The plan is to create a new DC/File Server and transfer all files across from the existing one before demoting and wiping it. I'll then install Hyper-V on the old one and create a new virtual DC on that for backup. I'll also setup DFS at the same time to make moving files a little easier in the future. How does that sound?

[danrhodes]

My only issue with that setup is the worry that the old server you plan to install Hyper-V on may not support Server 2008 or be able to handle any more than the one VM your are planning to install onto it?

 

Why not set the new server as a Virtual Host, install Server 2008 and the Hyper-V role, do a Physical to Virtual Conversion of the exisitng DC and its disks and have it run on the new Server as a VM, this should be a fairly seamless process and can be done within a few hours with no noticable changes for the end users, then flatten the old server and have that running as a RODC in case of a temporary failure of your PDC?

 

D

Edited May 24, 2010 by danrhodes

[steve]

If the disk is going to be used for storing alot of files (FileServer) then it may be better to connect a physical drive using Raw Device Mappings to a SAN rather than using a VHD.

 

D

 

This would give best performance (just). However if in the future you intend to have a fail over cluster with a SAN for shared storage you can't do this - the passthrough (RAW) disk wouldn't be available if the VM failed over too another host.

[djones]

My only issue with that setup is the worry that the old server you plan to install Hyper-V on may not support Server 2008 or be able to handle any more than the one VM your are planning to install onto it?

 

Yes, still need to check the state of play with that one.

 

Why not set the new server as a Virtual Host, install Server 2008 and the Hyper-V role, do a Physical to Virtual Conversion of the exisitng DC and its disks and have it run on the new Server as a VM, this should be a fairly seamless process and can be done within a few hours with no noticable changes for the end users, then flatten the old server and have that running as a RODC in case of a temporary failure of your PDC?

 

...that is fine but the existing DC is 2003 and I will eventually get rid of this completely. I did think of doing a P2V of it but I went against it as I decided it would be an additional step I didn't need if I just created a new virtual DC/File server, moved everything across and reloaded the old server again - hopefully as a virtual or physical 2008 DC.

 

In your suggestion above, what would you then have running on the new server, a P2V of the existing plus a physical DC on that hardware or a virtual DC or nothing else?

[danrhodes]

On your new server I would run server 2008 and Hyper-V only. Then I would P2V your old DC onto the new server running Hyper-V then flatten the old and make it a physical backup DC.

 

New Server (Hyper-V) ---> P2V of old DC

---> Other Virtual Servers

 

Old Physical DC - Flatten and use as a Backup DC.

 

D

[teejay]

Use fixed disks if you want the best performance, expanding disks are slower. See Checklist: Optimizing Performance on Hyper-V

[danrhodes]

Use fixed disks if you want the best performance, expanding disks are slower. See Checklist: Optimizing Performance on Hyper-V

 

Passthrough disks – The passthrough disk feature allows the guest operating system to bypass the Hyper-V Host file system and access the disk directly. Disks that are made available to guest operating systems via passthrough must be set to “offline” in the Hyper-V host to ensure that both the host and guest operating system do not attempt to access the disk simultaneously. The passthrough disk does offer a marginal performance advantage over other disk storage options but does not support certain functionality of virtual disks, such as Virtual machine snapshots and clustering support. Therefore use of the passthrough disk feature is not recommended in a BizTalk or SQL Server environment because the marginal performance benefits are more than offset by the missing functionality.

 

Still slightly better performance though :pbut no good if you want to use clustering or SQL - Shame

[teejay]

I'll rephrase, use fixed size vhd's for performance over expanding vhds and yes for most performance use passthrough ;-)

[tmcd35]

Use fixed disks if you want the best performance, expanding disks are slower. See Checklist: Optimizing Performance on Hyper-V

 

But how much slower?

 

The problem with fixed disk is two fold - 1) It's difficult to correctly guess how much space is going to be needed, unless 2) you dramtically over allocate space and end up with wasted, unused, unsable dead space on your SAN.

 

I've done it myself before. Allocated 12Gb to an OS drive and found it wasn't enough when upgrading OS or found the print logs taking up more space than expected. Although it's not impossible to increase the size of system disks - it's a right royal PITA.

 

As I said above, It's best not to use fixed disks unless you have a very specific and good reason - which depending on application may be performance related.

[danrhodes]

Exactly, it's very hard to guestimate the space requirements of the O/S and the services its going to be running, so for the slight loss of performance Dynamic Disks are the ay forward!

 

D

[teejay]

But how much slower?

 

The problem with fixed disk is two fold - 1) It's difficult to correctly guess how much space is going to be needed, unless 2) you dramtically over allocate space and end up with wasted, unused, unsable dead space on your SAN.

 

I've done it myself before. Allocated 12Gb to an OS drive and found it wasn't enough when upgrading OS or found the print logs taking up more space than expected. Although it's not impossible to increase the size of system disks - it's a right royal PITA.

 

As I said above, It's best not to use fixed disks unless you have a very specific and good reason - which depending on application may be performance related.

 

The performance hit is a minimum of 13% and can be well over 20%, so for a file server as per the op it could be quite a hit.

[danrhodes]

The performance hit is a minimum of 13% and can be well over 20%, so for a file server as per the op it could be quite a hit.

 

Ouch!!! Thats a very large chunk of performance there, where are those figures from, are they from M$?

 

1% performace hit for a heavily used file server can be dramatic never mind 13% - 20%, thats a killer!

 

D

[teejay]

Please see

Windows Server 2008 R2 & Hyper-V Server 2008 R2 RTM!!!! - Windows Virtualization Team Blog - Home - TechNet Blogs

 

 

So a fixed sized VHD will get to around 94% native perfomance, an expanding VHD 87%.

However, remember these are very optimistic maximum figures. In real life, especially for file servers etc where the data is usually expanding, you will see around a 20% performance hit.

[danrhodes]

Please see

Windows Server 2008 R2 & Hyper-V Server 2008 R2 RTM!!!! - Windows Virtualization Team Blog - Home - TechNet Blogs

 

 

So a fixed sized VHD will get to around 94% native perfomance, an expanding VHD 87%.

However, remember these are very optimistic maximum figures. In real life, especially for file servers etc where the data is usually expanding, you will see around a 20% performance hit.

 

Eeeek that's a really big hit for a F-S, so pass through or fixed is the best way, I think I'd be tempted to use PassThrough but if you don't have a SAN then maybe its got to be a Fixed Disk and over allocate what you may use. But then of course its wasted space if its not used.

 

D

Edited May 25, 2010 by danrhodes