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Re: [Xen-users] LVM or file storage?

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] LVM or file storage?
From: Boris Senker <senker@xxxxxxxxx>
Date: Mon, 12 Feb 2007 09:26:51 +0100
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thanks for your replies.

The reason loop appears to not generate as much load is that it is not
writing your data out to disk. It is cached in the memory by the loop
driver and only flushed periodically. Needless to say this is playing
russian roulette with your data - if you experiance an outage on Dom0
chances are that your guest filesystems will experiance *catastrophic*
data loss. Not even journalling in the guest FS will help you here
since the journall writes will simply be cached in memory in the loop

I'd definitely opt for reliable data storage, for the guests to be properly
restorable at any point.

If you want to compare performance of real block devices, vs a file
backed image use the blocktap driver instead of the loop driver.

eg Instead of


Use the path like


Will surely try.

Also, I'd recommend fully allocating the disk space for your file image,
rather than using sparse files - there is significant overhead involved
in extending the sparse files at runtime which can lead to unexpected
performance degradation. Sparse is fine for development/testing, but in
production you want non-sparse files.

Actually all file images were done with full allocations already. Saw the sparse vs. full
discussion somewhere before.

Not to mention some OSes don't support sparse files, like NetBSD.

For my machines I use LVM, it makes backups easier and giving a VM more space is a simple lvextend/growfs operation. You can always run LVM on top of RAID, at least that will give you some redundancy.

This is what attracted me towards LVM (especially in regards to that 150GB Samba partition). I am running LVM on top of raid already - the LV group and volumes were initially created on a /dev/md1.

But honesty, I am still not sure what storage type to incline towards.

Thanks again,


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