With reduced span cache time the block service cache
is no longer needed. We also don't need to fetch
changed block services from registry as we'll get
it as part of span fetches.
Things not done because probably disruptive:
* kmod filesystem string
* sysctl/debugfs/trace
* metrics names
* xmon instance names
Some of these might be renamed too, but starting with a relatively
safe set.
Also, split the timeouts for dentries and for stats. We generally
don't care if stats are out of dates, but dentries should be up
to date.
The code leaves various aspects to be desired:
* No attempt is made to only send stats when needed -- it is always
done. It might be a good idea to instead wait for the first two
stats to come back.
* Theres quite a bit of code duplication.
* It's pretty wasteful to have so many different packets for the
stats. It'd be much better to pack multiple requests and multiple
responses in single packets.
This could be done simply by allowing many requests to come
in the same packet (just one after the other would be fine),
and same for the responses. We can still use the protocol and
request id to keep track of things anyway.
Initial version really by Pawel, but many changes in between.
Big outstanding issues:
* span cache reclamation (unbounded memory otherwise...)
* bad block service detection and workarounds
* corrupted blocks detection and workaround
Co-authored-by: Paweł Dziepak <pawel.dziepak@xtxmarkets.com>
