On 5/1/2018 5:26 PM, Arvind N wrote:
In cases of rebuilding of an instance using a different image where the
image traits have changed between the original launch and the rebuild,
is it reasonable to ask to just re-launch a new instance with the new image?
The argument for this approach is that given that the requirements have
changed, we want the scheduler to pick and allocate the appropriate host
for the instance.
We don't know if the requirements have changed with the new image until
we check them.
Here is another option:
What if the API compares the original image required traits against the
new image required traits, and if the new image has required traits
which weren't in the original image, then (punt) fail in the API? Then
you would at least have a chance to rebuild with a new image that has
required traits as long as those required traits are less than or equal
to the originally validated traits for the host on which the instance is
currently running.
The approach above also gives you consistent results vs the other
approaches where the rebuild may or may not succeed depending on how the
original allocation of resources went.
Consistently frustrating, I agree. :) Because as a user, I can rebuild
with some images (that don't have required traits) and can't rebuild
with other images (that do have required traits).
I see no difference with this and being able to rebuild (with a new
image) some instances (image-backed) and not others (volume-backed).
Given that, I expect if we punt on this, someone will just come along
asking for the support later. Could be a couple of years from now when
everyone has moved on and it then becomes someone else's problem.
For example(from Alex Xu) ,if you launched an instance on a host which
has two SRIOV nic. One is normal SRIOV nic(A), another one with some
kind of offload feature(B).
So, the original request is: resources=SRIOV_VF:1 The instance gets a VF
from the normal SRIOV nic(A).
But with a new image, the new request is: resources=SRIOV_VF:1
traits=HW_NIC_OFFLOAD_XX
With all the solutions discussed in the thread, a rebuild request like
above may or may not succeed depending on whether during the initial
launch whether nic A or nic B was allocated.
Remember that in rebuild new allocation don't happen, we have to reuse
the existing allocations.
Given the above background, there seems to be 2 competing options.
1. Fail in the API saying you can't rebuild with a new image with new
required traits.
2. Look at the current allocations for the instance and try to match the
new requirement from the image with the allocations.
With #1, we get consistent results in regards to how rebuilds are
treated when the image traits changed.
With #2, the rebuild may or may not succeed, depending on how well the
original allocations match up with the new requirements.
#2 will also need to need to account for handling preferred traits or
granular resource traits if we decide to implement them for images at
some point...
Option 10: Don't support image-defined traits at all. I know that won't
happen though.
At this point I'm exhausted with this entire issue and conversation and
will probably bow out and need someone else to step in with different
perspective, like melwitt or dansmith.
All of the solutions are bad in their own way, either because they add
technical debt and poor user experience, or because they make rebuild
more complicated and harder to maintain for the developers.
--
Thanks,
Matt
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