Add check on total disk size before deploying

[Dmitry Tantsur]

Currently, the only thing we check is that instance image fits in a root partition. We need a check that all 3 possible partitions (ephemeral, swap and root) fit on a hard drive. Using test in review https://review.openstack.org/#/c/94620 I deducted the following formula: EPHEMERAL + SWAP + ROOT <= TOTAL_SIZE - 2 MiB.

What these 2 MiB account for:
1. Spacing for MBR before the first partition
2. Spacing after the last partition: parted can't have end of the partition after the end of the disk, i.e. `mkpart XX TOTAL_SIZE` won't work. Version of `parted` in Ubuntu Precise is still able to claim the trailing 1 MiB, if it's possible.
    This also accounts for GPT, which stores metadata also in the end.

[Lucas Alvares Gomes (lucasagomes)]

Copying the IRC chat as a reference here, I will to do some small investigation to see if that's still true:

<lucasagomes> dtantsur, afair, we need to fix some misunderstands with nova as well
<lucasagomes> for e.g
<lucasagomes> we have properties/local_gb, which represents the size of the _whole_ disk for ironic
<lucasagomes> but nova uses it as the size of the root_partition, independent of the ephemeral size or swap size
<lucasagomes> so what's passed for us is, size of the whole disk + eph + swap
<dtantsur> O_O
<lucasagomes> I think that is one reason why we don't have a sanity check within ironic
<lucasagomes> dtantsur, yeah, there's something odd around that area
<lucasagomes> is a bit grey area
<lucasagomes> dtantsur, we may want to calculate it on our driver tho

[Dmitry Tantsur (divius)]

Unassigned myself. Note that parted vs sfdisk issues should be solved first.

[Robert Collins (lifeless)]

So if this ever shows up, it means the scheduler did the wrong thing. This may be pointing at a scheduling algorithm bug.

[Dmitry Tantsur (divius)]

Robert, I guess we should be doing independent sanity checks (especially if we expect Ironic to be used w/o Nova)

[Pavlo Shchelokovskyy (pshchelo)]

Actually there already is a filter in Nova scheduler that does almost exactly that (except those extra 2 MiB) - ExactDiskFilter [1].

Moreover, it is included in default set of filters for IronicHostManager [2], but devstack (and probably others) do not use these default filters by default (sic) [3]

So probably the solution would be to update the ExactDiskFilter to account for extra 2MB (or create a modified variant of it) and start actually using it in scheduler filters.

[1] https://github.com/openstack/nova/blob/master/nova/scheduler/filters/exact_disk_filter.py#L23
[2] https://github.com/openstack/nova/blob/master/nova/scheduler/ironic_host_manager.py#L39
[3] https://github.com/openstack/nova/blob/master/nova/scheduler/ironic_host_manager.py#L45

[Dmitry Tantsur (divius)]

We should still double-check in case Nova is not used

[Launchpad Janitor (janitor)]

[Expired for Ironic because there has been no activity for 60 days.]