RE: [Xen-users] What are the advantages of running domU in a lvmlogical volume?

["Ross S. W. Walker" <rwalker@xxxxxxxxxxxxx>]

James Pifer wrote:
> 
> I've built my second xen server using SLES10SP1. I learned quite a bit
> on the first server with help from this list. This second xen 
> server is going to be used for hosting mostly windows domU's. 
> 
> I've followed the instructions here:
> http://ian.blenke.com/vmware/vmdk/xen/hvm/qemu/vmware_to_xen_hvm.html
> I've successfully converted two vmware images to xen domU's. It worked
> amazingly well!

Good news and thanks for the link!

> I have three questions. 
> 
> 1) The last part of instructions has you create a logical volume and dd
> the disk image to it. Based on my experience with the first server, I
> left a whole bunch of free space for doing this. 
> 
> But why do this? Why not just run the domU from the img file? I tested
> it on the ones I did and it seems to run fine. Just wondering if there
> are specific advantages for running from a logical volume. 

Performance.

If you run a domain from an image file with file:// as opposed to
a block device (whether it be LVM, raw disk, or raw partition) then
the domain manager is going to have to translate block io
instructions to vfs layer instructions which will have to pass
through the file system driver and page cache. IO is handled
synchronously for data-integrity so you don't see any caching
performance improvement unfortunately. :-(

If you use a block device with phy:// or tap:// the instructions
will be passed directly to the block device without translation.
This will allow arbitrarily sized io requests to be passed straight
down rather then broken into 4k chunks which is necessary for any io
on the vfs layer (memory mapped).

The advantages of using an image file though are, 1) size, with
growing image files it only uses the storage it needs, 2) portability,
it's easier to move image files around and convert them from
one format to another then raw storage. I'm sure someone can think
of more good reasons to use an image file.

> 2) For a windows domU, if you restart dom0, is the domU shutdown
> gracefully? I clicked shutdown from virt-manager and it looked like it
> just "cut power". 

No, not unless you have the Windows PV drivers installed.

> 3) What is a realistic expectation for how many domU's can run on a
> host? Obviously this is dependent on how they are being used, but if
> dom0 has two dual core xeons at 2.6 GHZ and 16 GB of RAM, could you
> conceivably run 5,6,or 7 windows servers with 2 GB of RAM 
> each? (let's assume NOT high transaction servers)

Yes, that is very reasonable. I run 10 guests on a similar config.

The two biggest limiting factors I have found out are, 1) physical
memory, 2) physical storage.

> As always, help is appreciated. 

More then happy to provide.

-Ross

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