[Gluster-devel] Volgen support for loading trace and io-stats translators at specific points in the graph

[Shyam]

On 05/30/2017 05:28 AM, Krutika Dhananjay wrote:
> You're right. With brick graphs, this will be a problem.
>
> Couple of options:
>
> 1. To begin with we identify points where we think it would be useful to
> load io-stats in the brick graph and unconditionally have
> glusterd-volgen load them in the volfile only at these places (not very
> useful if we want to load trace xl though. Plus, this again makes
> io-stats placement static).

I think this is needed (easier to get in), so +1 for this.

Additionally, if this is chosen, we may need specific triggers for each 
instance, to target measuring the io-stats. IOW, generic io-stats can 
measure below FUSE (as an example) and below server-protocol. Then, we 
may want to enable io-threads (assuming this is one instance on the 
brick that is a static placement), or POSIX (or both/all) specifically, 
than have them enabled by default when io-stats is turned on (which is 
the current behaviour).

Does this make sense?

>
> 2. Embed the trace/io-stats functionality within xlator_t object itself,
> and keep the accounting disabled by default. Only when required, the
> user can perhaps enable the accounting options with volume-set or
> through volume-profile start command for the brief period where they
> want to capture the stats and disable it as soon as they're done.

This is a better longer term solution IMO. This way there is no further 
injection of io-stats xltor, and we get a lot more control on this better.

Depending on time to completion, I would choose 1/2 as presented above. 
This is because, I see a lot of value in this and in answering user 
queries on what is slowing down their systems, so sooner we have this 
the better (say 3.12), if (2) is possible by then, more power to it.

>
> Let me know what you think.
>
> -Krutika
>
> On Fri, May 26, 2017 at 9:19 PM, Shyam <srangana at redhat.com
> <mailto:srangana at redhat.com>> wrote:
>
>     On 05/26/2017 05:44 AM, Krutika Dhananjay wrote:
>
>         Hi,
>
>         debug/io-stats and debug/trace are immensely useful for isolating
>         translators that are performance bottlenecks and those that are
>         causing
>         iatt inconsistencies, respectively.
>
>         There are other translators too under xlators/debug such as
>         error-gen,
>         which are useful for debugging/testing our code.
>
>         The trick is to load these above and below one or more suspect
>         translators, run the test and analyse the output they dump and debug
>         your problem.
>
>         Unfortunately, there is no way to load these at specific points
>         in the
>         graph using the volume-set CLI as of today. Our only option is to
>         manually edit the volfile and restart the process and be
>         super-careful
>         not to perform *any* volume-{reset,set,profile} operation and graph
>         switch operations in general that could rewrite the volfile,
>         wiping out
>         all previous edits to it.
>
>         I propose the following CLI for achieving the same:
>
>         # gluster volume set <VOL> {debug.trace, debug.io-stats,
>         debug.error-gen} <xl-name>
>
>         where <xl-name> represents the name of the translator above
>         which you
>         want this translator loaded (as parent).
>
>         For example, if i have a 2x2 dis-rep volume named testvol and I
>         want to
>         load trace above and below first child of DHT, I execute the
>         following
>         commands:
>
>         # gluster volume set <VOL> debug.trace testvol-replicate-0
>         # gluster volume set <VOL> debug.trace testvol-client-0
>         # gluster volume set <VOL> debug.trace testvol-client-1
>
>         The corresponding debug/trace translators will be named
>         testvol-replicate-0-trace-parent, testvol-client-0-trace-parent,
>         testvol-client-1-trace-parent and so on.
>
>         To revert the change, the user simply uses volume-reset CLI:
>
>         # gluster volume reset <VOL> testvol-replicate-0-trace-parent
>         # gluster volume reset <VOL> testvol-client-0-trace-parent
>         # gluster volume reset <VOL> testvol-client-1-trace-parent
>
>         What should happen when the translator with a
>         trace/io-stat/error-gen
>         parent gets disabled?
>         Well glusterd should be made to take care to remove the trace xl too
>         from the graph.
>
>
>
>         Comments and suggestions welcome.
>
>
>     +1, dynamic placement of io-stats was something that I added to this
>     spec [1] as well. So I am all for the change.
>
>     I have one problem though that bothered me when I wrote the spec,
>     currently brick vol files are static, and do not undergo a graph
>     change (or code is not yet ready to do that). So when we want to do
>     this on the bricks, what happens? Do you have solutions for the
>     same? I am interested, hence asking!
>
>     [1] Initial feature description for improved io-stats:
>     https://review.gluster.org/#/c/16558/1/under_review/Performance_monitoring_and_debugging.md
>     <https://review.gluster.org/#/c/16558/1/under_review/Performance_monitoring_and_debugging.md>
>
>