Xen users - Sep 2008 - Preferred Disk Image Format / Kernel Location

Hi everyone,

I''m working on a project right now focused on Xen and I''m
curious what
everyones experiences / preferences are regarding the following two  
aspects

Preferred Disk Partition Format
and
Location of the Kernel (ie Pygrub / Kernel outside an Image)

For the disk format I''ve tried all sorts of approach and I
can''t find
any specific benefits one way or the other, however I have been  
leaning more towards representing disk images as partition /dev/sda1.  
The other options of course are that I present the target as an entire  
drive, or as ''xvda''

The kernel location question seems more interesting to me. I''ve had a  
lot of success using pygrub within the different environments I''ve  
managed, and I personally prefer this method, but I''m interested in  
hearing arguments as to why this should or shouldn''t be done.


Thanks for your input!

-Trevor
_______________________________
Trevor Orsztynowicz
http://layerboom.com




_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

Hi,

Some comments inline.

On Tue, Sep 2, 2008 at 8:59 PM, Trevor Orsztynowicz
<trevoro@layerboom.com> wrote:
> Hi everyone,
>
> I''m working on a project right now focused on Xen and I''m
curious what
> everyones experiences / preferences are regarding the following two aspects
>
> Preferred Disk Partition Format
There are a lot of good choices here. From disk, partition images, to
partition, Logical Volumes, and even network-based solutions.

For a more detailed analysis I recommend a book that I was a co-author on.
> and
> Location of the Kernel (ie Pygrub / Kernel outside an Image)
>
For PV guests you have the option to have it outside, and sometimes
that is quite convenient, but for full virt (HVM) guests that is
typically not an option.

pygrub, as of Xen 3.3 has been deprecated in favor of a more secure
approach called PV-GRUB. Search for PV-GRUB at xen.markmail.org
for lots of good references on it, especially recently and also see
blog.xen.org for more details on it in a recent post by the developer.

> For the disk format I''ve tried all sorts of approach and I
can''t find any
> specific benefits one way or the other, however I have been leaning more
> towards representing disk images as partition /dev/sda1. The other options
> of course are that I present the target as an entire drive, or as
''xvda''
>
There is a lot of preference here and also quite a bit of people that
are doing benchmarking of different scenarios. Hopefully some people
will share their experiences and performance results here.
> The kernel location question seems more interesting to me. I''ve
had a lot of
> success using pygrub within the different environments I''ve
managed, and I
> personally prefer this method, but I''m interested in hearing
arguments as to
> why this should or shouldn''t be done.
>
>
I recommend looking into PV-GRUB and also understanding the security
implications of pygrub.

Hope that helps,
Cheers,
Todd


-- 
Todd Deshane
http://todddeshane.net
check out our book: http://runningxen.com

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

2008/9/3 Trevor Orsztynowicz <trevoro@layerboom.com>:

> I''m working on a project right now focused on Xen and I''m
curious what
> everyones experiences / preferences are regarding the following two aspects
> Preferred Disk Partition Format
Speaking only for myself, I prefer LVM LVs for each virtual disk,
either locally or served up over iSCSI
> Location of the Kernel (ie Pygrub / Kernel outside an Image)
I prefer pygrub as it keeps the domU as similar to a non-virtual
machine as possible, the possible security concerns over pygrub should
be largely addressed by pvgrub.

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

Trevor Orsztynowicz <trevoro@layerboom.com>
writes:
> I''m working on a project right now focused on Xen and I''m
curious what
> everyones experiences / preferences are regarding the following two
> aspects
> 
> Preferred Disk Partition Format
> and
> Location of the Kernel (ie Pygrub / Kernel outside an Image)
I have a strong preference for keeping the kernel in the image.  I understand
the security concerns, but I consider it worth it to get the ease of 
management and flexibility.   Of course, now that 3.3 out, I''m going to
move all my stuff over to pvgrub.  Either way, allowing the DomU administrator
to change the kernel allows you to separate the ''virtual server
administrator'' and ''hardware/Dom0 administrator''
roles much better than
keeping the kernel in the Dom0.
> For the disk format I''ve tried all sorts of approach and I
can''t find
> any specific benefits one way or the other, however I have been
> leaning more towards representing disk images as partition /dev/sda1.
> The other options of course are that I present the target as an entire
> drive, or as ''xvda''
Largely preference.  Personally, I like to give each DomU 2 LVM  partitions
from the dom0 point of view, the partition is named after the DomUs name, 
and the second partition is domuname_boot.  this lets me put the PyGrub stuff
on it''s own partition (meaning that a user can more freely screw up her
main partition and then reboot into PyGrub, and choose the rescue image
to fix the problem without pestering me)   From within the DomU, the 
user sees xvda and xvdb (or sda and sdb, depending)  so the user can 
partition xvdb however they like.  

Long term, I''d like to integrate some kind of PyPXEboot type thing into
pvgrub, so that a user can boot her own kernels off disk, or alternately 
hit a key and download pxe kernels (to get a rescue/install image or the 
like, assuming they blew away the disk) but I''ve not spent the time to
set
that working.

but I know people who prefer to partition sda in the Dom0 as
/dev/sda5 for vm 1, /dev/sda6 for vm 2, etc.   Personally, I think that
is difficult to manage, but again, it''s largely preference.  

The real performance differences are between file: vs. phy: vs nfs or iscsi 
(or AoE or gnbd or other ''load the filesystem from within the DomU over
the
network'')  devices.  phy: as a general rule, works well and is faster
than
most anything else, but doesn''t do live migration.   Network file
systems are
all pretty slow until you get into the ''silly expensive''
range.

as for file: devices, they are easy, fine for testing.  some people 
find files easier to deal with organizationally.  tap:// devices are a good
answer for those people, but tap: lacks many of the advantages of files.
tap: is picky about the filesystem and type of file... you aren''t going
to be able to tap: a file over NFS, and tap: is weird with sparse files 
(see qcow, but I think sparse files are generally a bad idea.)    
> The kernel location question seems more interesting to me. I''ve
had a
> lot of success using pygrub within the different environments I''ve
> managed, and I personally prefer this method, but I''m interested
in
> hearing arguments as to why this should or shouldn''t be done.
pvgrub overcomes most of the reasons why you wouldn''t want to use
PyGRUB.
the only time when I think it might be easier to keep the kernels in the
Dom0 is if you were using a kernel with no modules, and all your DomUs
always used the same kernel.  In that case, it could be easier to keep 
it in the Dom0.  But in most situations where I''ve used Xen, keeping
the
kernel in the DomU is much easier.  

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users