documentation still mentions relayfs, but I believe debugfs is correct.
Small fix attached.
Usage
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-$ blktrace -d <dev> [ -r relay_path ] [ -o output ] [ -k ] [ -w time ]
+$ blktrace -d <dev> [ -r debug_path ] [ -o output ] [ -k ] [ -w time ]
[ -a action ] [ -A action mask ]
-d Use specified device. May also be given last after options.
- -r Path to mounted relayfs, defaults to /relay.
+ -r Path to mounted debugfs, defaults to /debug.
-o File(s) to send output to.
-D Directory to prepend to output file names.
-k Kill running trace.