Re: [Xen-users] Re: Cannot boot with LVM inside Xen DomU

[Oliver Wilcock <oliver@xxxxxxx>]

Sorry about the late reply.

Resizing of the root disk.  Here are some thoughts:
I previously shared that you may run into trouble when upgrading if the 
root (/) is the entire disk.  In my experience the SUSE installer was 
confused (it couldn't find the existing installation) and had to be 
tricked.  Consequently I now put root on /dev/xvda1.
It doesn't sound right that you can resize with xm block-attach.  You 
may want to re-read whatever you are referring to.
Why would you need to make your root disk bigger?  It is probably 
smarter to put the data that grows on separate LVM backed storage. 
Perhaps you need to assign separate LVs to /home /var /opt.  For 
relatively simple guests I would typically create an LV for swap, var 
and home and mount those as disks 
(phy:/dev/raid10x6/guestservername-home,xvdc,w).  You can do online 
expansion of the LV which appears as a larger xvdc device.
In dom0: lvresize -L?GB /dev/raid10x6/guestservername-home

Performance penalty of nested LVM: I'm sure there is an impact (a few 
percent probably, I've never measured it).  You might still want to do 
it though.  I've done it once for the ability to create LV snapshots 
inside the guest as part of the backup strategy.  I didn't do it on root 
(/) though.  Just the mount points where I kept the databases and log files.

On 2011-07-11 14:47, Prakhar Srivastava wrote:
> Hi,
snip

> I have one more query. As stated by Fazar in his last reply,
> Redhat/CentOS DomU have by default LVM within which allows them to
> increase the root disk online by xm block-attach command. However, I
> have read in one of the Xen list discussion that using LVM as DomU
> backend and also within DomU is not a great idea in terms of
> performance. I want to have a similar kind of setup and believe that
> having the flexibility of resizing root disk online is a great plus in a
> virtualized environment. What are your thoughts on this ??
>
> Thanks,
> Prakhar
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