combined per-XYZ I/O usage stats

[milianw]

Right now when I use the lttng-iousagetop command, I get useful Per-process I/O, Per-file I/O and Block I/O reports. What I'm missing is a way to combine this, to get e.g. the per-process I/O for every file and ideally also for every block.

That would allow one to figure out which file was written to by what process, and on what block that file lives.

[jdesfossez]

The per-process I/O report is already the sum of every I/O operation made to all the FDs belonging to a process, and the per-file report combines all the I/O made by every process that have opened this file. I am not sure what you mean exactly, could you maybe provide an example ?

For the blocks, it is more tricky, because we do not have a definitive mapping between syscalls and block operations, especially with caching/writeback. It really is a best effort here because we do not have all the instrumentation required.