blktrace: disallow -o when using multiple devices

Document that "-o" does not work when specyfing multiple devices to blktrace, also: enforce this by stopping blktrace when one tries do do this.

The technical reason why "-o" doesn't work with multiple devices is because we use multiple threads of execution - one per device/CPU pair - and each of them opens a file named "<prefix>.blktrace.<cpu>". With the "-o" all of the"<prefix>" values are the same - so multiple threads open the same file and try to do output. Not good. Without the "-o" we get unique files named: "<device>.blktrace.<cpu>" - as the tuple (<device>,<cpu>) is unique.

-Wouldn't it be a good idea to just add<prefix> before the filename(s) that would have been created in either the single or multiple device case? I.e. in the multiple device case, how about "<prefix>.<device>.blktrace.<cpu>"? That way the program always does what the user wants, no special case documentation required.

Quoted 5 lines · Brett Russ

-Wouldn't it be a good idea to just add<prefix> before the filename(s) that would have been created in either the single or multiple device case? I.e. in the multiple device case, how about "<prefix>.<device>.blktrace.<cpu>"? That way the program always does what the user wants, no special case documentation required.

I just now read in full your second email on the original thread where you mentioned doing this solution, sorry for the redundancy. You expressed reservations about it, wondering what else it might break. I'd suggest we make the change and test it, rather than having a "-o" option with limited utility? I imagine the biggest question mark is how blkparse handles it.

-Brett

Quoted 14 lines · Brett Russ

Quoted 5 lines · Brett Russ

-Wouldn't it be a good idea to just add<prefix> before the filename(s) that would have been created in either the single or multiple device case? I.e. in the multiple device case, how about "<prefix>.<device>.blktrace.<cpu>"? That way the program always does what the user wants, no special case documentation required.

I just now read in full your second email on the original thread where you mentioned doing this solution, sorry for the redundancy. You expressed reservations about it, wondering what else it might break. I'd suggest we make the change and test it, rather than having a "-o" option with limited utility? I imagine the biggest question mark is how blkparse handles it.

Quoted 1 line

-Brett

I'm not so sure about this Brett - people may have scripts out there that depend on the current semantics. I'd be OK with doing it if Jens thought this was a minor concern...