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Re: [Xen-users] Partition vs disk images

To: "Fajar A. Nugraha" <fajar@xxxxxxxxx>
Subject: Re: [Xen-users] Partition vs disk images
From: Rudi Ahlers <Rudi@xxxxxxxxxxx>
Date: Tue, 29 Apr 2008 08:21:25 +0200
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Fajar A. Nugraha wrote:
Rudi Ahlers wrote:
Joseph L. Casale wrote:
This is for use at home, so in reality it probably doesn't matter -- I just don't want to make any *really* stupid moves.

I would seriously recommend LVM, its so flexible
But is there really a performance difference between LVM & file based VM's?

Yes, but depending on how you use it, the effect might not be what you expect.

Example 1 :
- you have 16G mem : 4G for domU, 12 G for dom0
- 1 domU running, with 4G root filesystem
In this scenario, you'd most likely find file-based VM perform better since dom0 will cache domU's root fs.

Example 2 :
- you have 8G mem : 7G for for 7 domUs (1G each), 1G for dom0
- each domU comes with 4G root filesystem
In this scenario, the performance of file-based VM will be lower compared to LVM-based VM, due to dom0's filesystem operation overhead (including journaling).

There are other pros and cons for file vs LVM. Personally, I prefer LVM with each LV on dom0 maps to a partition on domU (hda1,sda1, etc) because :
- I can easily convert a domU into a physical machine if necessary
- I can easily mount domU's fs directly on dom0 (e.g for troubleshooting purposes)
- I can get per-LV I/O statistic from dom0 using tools like iostat
- I can use LVM snapshot to backup domU's fs without shutting it down

This setup also means that I can't create a VM using frontends like virt-manager (which maps file/LV/partition on dom0 to a disk on domU), but that doesn't matter since I prefer to create VMs manually using prebuilt-template anyway :D



ok, what you're saying does make sense, so....

- If you say you can easily convirt a VPS to a physical machine, do you just move the LV to a new machine?
- I can mount image based domU as well :)
- But doesn't xen have IO stats as well?
- I prefer not to use snapshot backups, since it makes it more difficult to restore just one or two files. We mainly use cPanel & Plesk for our hosting, so I use the control panel's backup over FTP instead


Kind Regards
Rudi Ahlers
CEO, SoftDux

Web:   http://www.SoftDux.com
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