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Re: [Xen-users] Why is xen allocating entire disk/file?

To: "Fajar A. Nugraha" <fajar@xxxxxxxxx>
Subject: Re: [Xen-users] Why is xen allocating entire disk/file?
From: Charlie Reddington <charlie.reddington@xxxxxxxxx>
Date: Wed, 28 Jan 2009 23:41:28 -0600
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On Jan 28, 2009, at 11:30 PM, Fajar A. Nugraha wrote:

On Thu, Jan 29, 2009 at 12:20 PM, Charlie Reddington
<charlie.reddington@xxxxxxxxx> wrote:
# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1             446G   43G  381G  10% /
tmpfs                 3.9G     0  3.9G   0% /dev/shm

Right here alone, has me kind of sketcy. Total - 446G - 43G should = 403G

No comment on that one :)


But in var/lib/xen/images.....

# du -sh *
9.8G    ns1.img
9.8G    ns2..img
119M    shared01.img
20G             www.img

Doing the command as you say.....

9.8G -rwxrwxr-x 1 root root 9.8G Jan 28 15:18 ns1.img
9.8G -rwxrwxr-x 1 root root 9.8G Jan 28 15:18 ns2.img
134M -rwxrwxr-x 1 root root  98G Jan 28 15:18 shared01.img
20G -rwxrwxr-x 1 root root  20G Jan 28 15:18 www.img


So it doesn't use 100G right away, right?
Both "du -sh" and "ls -lash" (leftmost column) shows space actually
used. It can be different from the right column if it's created as
sparse file (in your case, "not to allocate the entire disk yet").

I'm just overall kind of confused how this is working. I should in theory be able to over allocate a number of vm's, so that when I get close to
running out, I can just add more disk space as cash allows.

Yes, it should work like that. Note however that using sparse file has
the downside of :
- lower I/O performance initially (i.e. only when it has to allocate
disk space previously used)
- there might be a condition when you run out of space (e.g. when all
domU unexpectedly needs more space) which can lead to unpredictable
result.

Regards,

Fajar

Thanks for the reply. I'm just trying to get the most out of this stuff as we are not. We'll be adding a lot more storage in the future, but are working on getting things going. I just want everything to have the ability to expand as it needs, and we should have everything in place before running out of disk space is an issue.

So, since it's calling the file 100gb, but only use 20, will I run into any problems on the host system thinking it's out of memory? That's really probably my only concern. If I setup 10vms and I'm over by a 100gb or so, will the host os complain?

Thanks,

Charlie

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