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

Aravinda <avishwan@xxxxxxxxxx> · Fri, 1 Sep 2017 12:27:50 +0530



    On 09/01/2017 10:57 AM, Amar Tumballi
      wrote:

    
      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.
      
    
    How about using UDP to push data from Gluster processes?

    
    - Signal handling only required while reloading vol file when
    metrics enabled/disabled

    - Push it to pre-configured UDP address(socket file), if listener
    exists it will capture metrics else UDP message is lost

    - Will not affect the io performance since it is asynchronous and
    error is not checked by the sender.

    - If metrics is lost, will not impact any data/io. We don't need
    crash consistency or high accuracy while collecting metrics.

    - Receiver can receive data fast without blocking incoming data and
    can produce outputs in different formats asynchronously.

    
    Usage:

    - Enable metrics using volopt

    - Start udp server(Metrics receiver), Example 
    ./gluster-metrics-receiver (socket file should be predefined say
    `/var/run/gluster/metrics.socket`)

    
    Limitations:

    Similar to `strace` command, running two receiver same time is not
    possible. It is possible to run multiple receivers by having
    different socket address for each process/pid. For example,
    `metrics.<pid>.socket` and receiver will listen using
    `./gluster-metrics-receiver -p <pid>`

    
        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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    -- 
regards
Aravinda VK
http://aravindavk.in

  
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