Hi Madhuri-
For this purpose, we would need to change a trait of the server’s flavor in Nova. This trait is mapped to a deploy step in Ironic which does some operation(change BIOS config and reboot in this use case).____
If your trait was something that wasn't tracked in the flavor (or elsewhere in the instance's db record), you could just update it directly in placement. Then you'd have to figure out how to make ironic notice that and effect the change. (Or perhaps the other way around: tell ironic you want to make the change, and it updates the trait in placement as part of the process.)
In Nova, the only API to change trait in flavor is resize whereas resize does migration and a reboot as well.____
In short, I am  looking for a Nova API that only changes the traits, and trigger the ironic deploy steps but no reboot and migration. Please suggest.____
It's inconvenient, but I'm afraid "resize" is the right way to get this done, because that's the only way to get the appropriate validation and changes effected in the general case. Now, there's a spec [1] we've been talking about for ~4.5 years that would let you do a resize without rebooting, when only a certain subset of properties are being changed. It is currently proposed for "upsizing" CPU, memory, and disk, and adding PCI devices, but clearly this ISS configuration would be a reasonable candidate to include. In fact, it's possible that leading the charge with something this unobtrusive would reduce some of the points of contention that have stalled the blueprint up to this point. Food for thought. Thanks, efried [1] https://review.opendev.org/#/c/141219/