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Hi Chris,

Yes we used qcow image. However I still have something not clear. What I know is that:

- disk_available_least = free_disk - disk_over_commit
- disk_over_commit = int(virt_size) - physical_disk_size

Before launching: 
- free_disk = 1025
- disk_available_least = 1016

If we use the normal flavor with 600Gb disk and no ephemeral., the result is good with both above value subtract 600Gb.
If we use the flavor without disk but 600Gb ephemeral, the result of disk_available_least = -184. The value of free_disk is still good with 425Gb. In this case, it seems that the value of disk_over_commit is 609Gb that is calculated from virt_size and physical_disk_size. What I think is the virt_size here is 600Gb and physical_disk_size is couple of Gb. How can the value 609Gb appear? I agree that the backing_file of ephemeral_disk is sparse and 600Gb.

Best,

Tuan
-----Original Message-----
From: Chris Friesen [mailto:chris.friesen at windriver.com] 
Sent: August 03, 2016 18:34
To: openstack-dev at lists.openstack.org
Subject: Re: [openstack-dev] [Nova] Nova compute reports the disk usage

On 08/03/2016 09:13 AM, Tuan Luong wrote:
> Hi Jay, Thank you for the reply. Actually, I did ask this question to 
> make sure that everything is still going well with disk usage report 
> of Nova. We had the problem related to wrong report (IMHO it is 
> wrong). The situation is
> below:
>
> - We launch an instance with 600Gb of ephemeral disk of flavor without 
> specifying extra ephemeral disk. The total free_disk before launching 
> is 1025Gb, the total disk_available_least is 1016Gb. - After 
> successful launching, what we have is the value of disk_available_disk 
> = -184Gb therefore we could not launch the next instance.
>
> It seems like the value of disk_available_disk decreases 2*600Gb when 
> I think it should be 600Gb. The value of free_disk after launching is 
> calculated well. I would not believe that the disk_over_commit in this case is 600Gb.
> Otherwise, we also check the instance that indeed it has only 600Gb 
> mounted as vdb.

Assuming you're using qcow, the maximum disk space consumed by a qcow disk is "size of backing file" + "size of differences from backing file".  Thus, if you have a single instance using a given backing file, the worst-case consumption is actually twice the size of your disk.

I think the backing file for an ephemeral disk is actually a sparse file, but on a cursory examination it will appear to be consuming 600GB.

Chris

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