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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