Add a plugin to collect bcache perf stats

[pponnuvel]

Collecting perf stats on bcache [0] [1] is useful to identify performance problems if they're deployed as a caching layer.
Useful stats are:

• /sys/block/bcache*/bcache/{cache_mode,dirty_data,io_errors,sequential_cutoff,state,writeback_percent,stats_hour/{bypassed,cache_{hit_ratio,hits,misses}},cache/internal/copy_gc_enabled}

• /sys/fs/bcache/*/{block_size,bucket_size,cache_available_percent,congested_*_threshold_us}

• /sys/fs/bcache/*/{*,stats_*/*,internal/*,bdev*/*,bdev*/stat_*/*}

I plan to work on adding a new plugin to collect these.

[0] https://wiki.archlinux.org/index.php/bcache
[1] https://bcache.evilpiepirate.org/

[hillpd]

This data would definitely be useful.

Be mindful of lp#1840043 [0]. It is an older issue where stat collection can severely impact system performance. The plug-in may need to check and prevent stat collection for exposed kernels.

[0] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1840043

[bmr-cymru]

Executing the stat collection can be gated by a predicate - we just need the right criteria for when to exclude the collection. The SoSPredicate supports various checks that are used to determine whether or not a command should run.

[pponnuvel]

@hillpd Thanks, Dan, for the note. priority_stats isn't in the list of the stats I planned to collect on bcache (which degrades performance when queried). So we're OK.

[pponnuvel]

@bmr-cymru It'd be useful if bcache are deployed as a caching device. I am thinking of IndependentPlugin with files set one of the sysfs entry something like as: b6a7a81

(I haven't tested b6a7a81 yet - will make a PR after test).