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

[Krutika Dhananjay]

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.

-Krutika
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