[bcache] Add a new plugin for bcache

[pponnuvel]

bcache is used as a caching device (typically an SSD) for
HDDs; bcache stats are useful to identify performance problems.

Closes: #2378
Resolves: #2384

Signed-off-by: Ponnuvel Palaniyappan ponnuvel.palaniyappan@canonical.com

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[pponnuvel]

I have tested the changes using a setup that uses 2 x 1:1 mapping (1 bcache device per backing device).

The sos report file (tar.xz) size increase with this setup is ~36KB. The files gathered for bcache plugin are text files, so even if the system has many number of bcache devices, it's not going to massively increase the sos report.

Attaching a sample sos report here:
sosreport-bcache-test.zip

(had to zip a tar.xz file because github doesn't allow tar .xz file ;)

[TurboTurtle]

General ack overall. Just 2 minor notes.

[slashdd]

Would it be best to execute add_copy_spec() iff the bcache module is loaded via SosPredicate' kmods ?

[pponnuvel]

Thanks, @slashdd. Gated via a predicate (I believe having files also ensures the same).

[pmoravec]

ACK, relying on the reporter's knowledge that the collecting files other than priority_stats can't harm a similar way like priority_stats would.

[hillpd]

A small nit, recommend changing to:
degrade performance on older kernels.

I'm suggesting this change just to be clear that the issue has been fixed in recent kernels.

[pponnuvel]

Hi Dan,
My understanding is that performance is much improved with that fix, but it still has some impact. I am referring this message in that thread: https://lkml.org/lkml/2019/8/14/636

[hillpd]

Ah, you are right. There is still a minor performance hit due to locking.