nvme: add support for nvme top

[ikegami-t]

This is to monitor the system changes.

[ikegami-t]

Will do consider to change the PR changes to add --continue or --delay option instead of the nvme top command. (Or just rename to nvme watch command instead.)

[ikegami-t]

Hi @igaw, @shroffni and @martin-belanger,

Just updated the PR changes to add --delay option instead of nvme top command (but not --continuous option) as mentioned by the matrix chat.

[shroffni]

After looking at the code, it does not seem appropriate to add a --delay option at the global configuration level.

The main issue is that a global --delay does not make sense for all nvme-cli commands. For example, commands such as create-ns or delete-ns should not accept a --delay option. If create-ns were invoked with a non-zero delay, it could repeatedly create namespaces in a loop until the device runs out of space. Similarly, commands like delete-ns, set-feature, format, and several others have no meaningful semantics when executed periodically with a delay.

If the goal is simply to monitor or repeatedly execute a command, the existing watch utility already provides this functionality. While watch may not be the most efficient solution, it is sufficient for user-space tooling and works reasonably well today.

If we still believe that a --delay option is necessary, then it should be added selectively, only to those nvme-cli sub-commands where periodic execution actually makes sense. One such example is show-topology, and this could also be useful for upcoming tools like nvme-top.

In summary, rather than introducing a global --delay option, it would be better to:

• Identify the specific sub-commands where delayed or periodic execution is meaningful, and
• Add support for --delay only to those commands.

[igaw]

I agree with @shroffni's arguments.

[ikegami-t]

Just changed the PR commit to add the option only for the show-topology command at first. Thank you.