Xen users - Apr 2008 - What are the advantages of running domU in a lvm logical volume?

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!

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. 


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". 


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)

As always, help is appreciated. 

Thanks,
James


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On Thu, 2008-04-10 at 10:14 -0400, Dan VerWeire wrote:
> 2) I haven''t used virt-manager but I assume what you are
experiencing
> is correct. Without some PV drivers installed into the Windows DomU,
> it doesn''t receive the signals to shut down correctly. There are
some
> GPL PV drivers being developed. Search this mailing list for details.
> They are being developed by James Harper.
> 
I can''t really see needing to restart dom0 very often, but if you ever
have to, do you manually shut down the windows OSes?
> 3) I currently have 4 Windows 2000 Server domUs running in a server
> that has 8gigs of ram and dual Xeon quad cores running @ 1.86GHz. Two
> of these VMs are high transaction database servers. One is MSSQL and
> the other is Pervasive. Also running a terminal server and a web
> server. All of these VMs were migrated from physical servers and we
> are experiencing higher stability that we were before the
> consolidation.
> 
Very good information. Thanks!

James


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On Thu, Apr 10, 2008 at 10:50 AM, James Pifer <jep@obrien-pifer.com>
wrote:
> On Thu, 2008-04-10 at 10:14 -0400, Dan VerWeire wrote:
>  > 3) I currently have 4 Windows 2000 Server domUs running in a server
>  > that has 8gigs of ram and dual Xeon quad cores running @ 1.86GHz. Two
>  > of these VMs are high transaction database servers. One is MSSQL and
>  > the other is Pervasive. Also running a terminal server and a web
>  > server. All of these VMs were migrated from physical servers and we
>  > are experiencing higher stability that we were before the
>  > consolidation.
>  >
>
>  Very good information. Thanks!
very nice datapoint. care to elaborate on HD setup? i guess this is
using PV drivers, but the hardware side is also very significant.



-- 
Javier

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On Thu, Apr 10, 2008 at 12:21 PM, Javier Guerra <javier@guerrag.com>
wrote:
> On Thu, Apr 10, 2008 at 10:50 AM, James Pifer <jep@obrien-pifer.com>
> wrote:
> > On Thu, 2008-04-10 at 10:14 -0400, Dan VerWeire wrote:
> >  > 3) I currently have 4 Windows 2000 Server domUs running in a
server
> >  > that has 8gigs of ram and dual Xeon quad cores running @
1.86GHz. Two
> >  > of these VMs are high transaction database servers. One is MSSQL
and
> >  > the other is Pervasive. Also running a terminal server and a web
> >  > server. All of these VMs were migrated from physical servers and
we
> >  > are experiencing higher stability that we were before the
> >  > consolidation.
> >  >
> >
> >  Very good information. Thanks!
>
> very nice datapoint. care to elaborate on HD setup? i guess this is
> using PV drivers, but the hardware side is also very significant.


We have 8 250gig Seagate SATA drives (ST3250410AS) in RAID 10 on a 3ware
9650SE-8LPML raid controller. We are NOT using PV drivers yet. Performance
is pretty good. I''m sure it will be better once the PV drivers mature.
Could
get some benchmarks if you are interested.


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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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On April 10, 2008 06:47 am 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!
>
> 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.
Image files make things more portable (just copy the file around), but 
slow things down as you have a filesystem in a partition (domU) in a file 
on a filesystem (dom0) on a block device (phy).

LVM partitions make things less portable (you have to clone the partition 
instead of just copying a file) but speed things up as you just have a 
filesystem in a partition (domU) on a block device (phy).

Or something along those lines.
> 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".
Connect to the Windows domU via rdesktop or vnc and watch what happens. :)
> 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)
I haven''t benchmarked it yet, or run any real performance checks, but
my
rule-of-thumb so far has been to use 1 Ghz of CPU per VM and 768 MB of 
RAM per VM as a minimum.  So, personally, I wouldn''t run more than 6
VMs
on your system without doing some benchmarking to see how thing run.

I only have two Xen boxes setup up to date:
  * 2x single-core Opteron 2 GHz CPUs w/4 GB RAM (64-bit Debian Etch dom0) 
running 4 PV VMs all running 64-bit Debian Etch domUs (these are web, 
subversion, samba, and cups servers)

  * 2x dual-core Opteron 2 GHz CPUs w/8 GB RAM (64-bit Ubuntu Hardy) 
running a mixture of PV and HVM VMs running with various FreeBSD, Linux, 
and Windows domUs, with up to 7 running at a time (this is just a test 
server at the moment)

-- 
Freddie Cash
fjwcash@gmail.com

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