[Gluster-Maintainers] Metrics: and how to get them out from gluster

Fri Sep 1 05:27:37 UTC 2017

Disclaimer: This email is long, and did take significant time to write. Do
take time and read, review and give feedback, so we can have some metrics
related tasks done by Gluster 4.0

---
** History:*

To understand what is happening inside GlusterFS process, over the years,
we have opened many bugs and also coded few things with regard to
statedump, and did put some effort into io-stats translator to improve the
gluster's monitoring capabilities.

But surely there is more required! And some glimpse of it is captured in
[1], [2], [3] & [4]. Also, I did send an email to this group [5] about
possibilities of capturing this information.

** Current problem:*

When we talk about metrics or monitoring, we have to consider giving out
these data to a tool which can preserve the readings in a periodic time,
without a time graph, no metrics will make sense! So, the first challenge
itself is how to get them out? Should getting the metrics out from each
process need 'glusterd' interacting? or should we use signals? Which leads
us to *'challenge #1'.*

Next is, should we depend on io-stats to do the reporting? If yes, how to
get information from between any two layers? Should we provide io-stats in
between all the nodes of translator graph? or should we utilize
STACK_WIND/UNWIND framework to get the details? This is our *'challenge #2'*

Once the above decision will be taken, then the question is, "what about
'metrics' from other translators? Who gives it out (ie, dumps it?)? Why do
we need something similar to statedump, and can't we read info from
statedump itself?". But when we say 'metrics', we should have a key and a
number associated with it, statedump has lot more, and no format. If its
different from statedump, then what is our answer for translator code to
give out metrics? This is our *'challenge #3*'

If we get a solution to above challenges, then I guess we are in a decent
shape for further development. Lets go through them one by one, in detail.

** Problems and proposed solutions:*

*a) how to dump metrics data ?*

Currently, I propose signal handler way, as it will give control for us to
choose what are the processes we need to capture information on, and will
be much faster than communicating through another tool. Also considering we
need to have these metrics taken every 10sec or so, there will be a need
for efficient way to get this out.

But even there, we have challenges, because we have already chosen both
USR1 and USR2 signal handlers, one for statedump, another for toggling
latency monitoring respectively. It makes sense to continue to have
statedump use USR1, but toggling options should be technically (for
correctness too) be handled by glusterd volume set options, and there
should be a way to handle it in a better way by our 'reconfigure()'
framework in graph-switch. Proposal sent in github issue #303 [6].

If we are good with above proposal, then we can make use of USR2 for
metrics dump. Next issue will be about the format of the file itself, which
we will discuss at the end of the email.

NOTE: Above approach is already implemented in 'experimental' branch,
excluding handling of [6].

*b) where to measure the latency and fops counts?*

One of the possible way is to load io-stats in between all the nodes, but
it has its own limitations. Mainly, how to configure options in each of
this translator, will having too many translators slow down operation ?
(ie, create one extra 'frame' for every fop, and in a graph of 20 xlator,
it will be 20 extra frame creates for a single fop).

I propose we handle this in 'STACK_WIND/UNWIND' macros itself, and provide
a placeholder to store all this data in translator structure itself. This
will be more cleaner, and no changes are required in code base, other than
in 'stack.h (and some in xlator.h)'.

Also, we can provide 'option monitoring enable' (or disable) option as a
default option for every translator, and can handle it at xlator_init()
time itself. (This is not a blocker for 4.0, but good to have). Idea
proposed @ github #304 [7].

NOTE: this approach is working pretty good already at 'experimental'
branch, excluding [7]. Depending on feedback, we can improve it further.

*c) framework for xlators to provide private metrics*

One possible solution is to use statedump functions. But to cause least
disruption to an existing code, I propose 2 new methods. 'dump_metrics()',
and 'reset_metrics()' to xlator methods, which can be dl_open()'d to xlator
structure.

'dump_metrics()' dumps the private metrics in the expected format, and will
be called from the global dump-metrics framework, and 'reset_metrics()'
would be called from a CLI command when someone wants to restart metrics
from 0 to check / validate few things in a running cluster. Helps
debug-ability.

Further feedback welcome.

NOTE: a sample code is already implemented in 'experimental' branch, and
protocol/server xlator uses this framework to dump metrics from rpc layer,
and client connections.

*d) format of the 'metrics' file.*

If you want any plot-able data on a graph, you need key (should be string),
and value (should be a number), collected over time. So, this file should
output data for the monitoring systems and not exactly for the
debug-ability. We have 'statedump' for debug-ability.

So, I propose a plain text file, where data would be dumped like below.

```
# anything starting from # would be treated as comment.
<key><space><value>
# anything after the value would be ignored.
```
Any better solutions are welcome. Ideally, we should keep this friendly for
external projects to consume, like tendrl [8] or graphite, prometheus etc.
Also note that, once we agree to the format, it would be very hard to
change it as external projects would use it.

I would like to hear the feedback from people who are experienced with
monitoring systems here.

NOTE: the above format works fine with 'glustermetrics' project [9] and is
working decently on 'experimental' branch.

------

** Discussions:*

Let me know how you all want to take the discussion forward?

Should we get to github, and discuss on each issue? or should I rebase and
send the current patches from experimental to 'master' branch and discuss
in our review system?  Or should we continue on the email here!

Regards,
Amar

References:

[1] - https://github.com/gluster/glusterfs/issues/137
[2] - https://github.com/gluster/glusterfs/issues/141
[3] - https://github.com/gluster/glusterfs/issues/275
[4] - https://github.com/gluster/glusterfs/issues/168
[5] - http://lists.gluster.org/pipermail/maintainers/2017-August/002954.html
(last email of the thread).
[6] - https://github.com/gluster/glusterfs/issues/303
[7] - https://github.com/gluster/glusterfs/issues/304
[8] - https://github.com/Tendrl
[9] - https://github.com/amarts/glustermetrics

-- 
Amar Tumballi (amarts)
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