Revisit option name that turns on xlators

[prashanthpai]

The xlator option table can also contain options that load (add to graph) or turn on an xlator. For example, to turn on the barrier xlator, as of today, one has to set option named barrier.barrier to on. This should rather simply be just barrier. IOW, we need a way to identify such options. One approach is to let xlators themselves define which option in option table is a "gobal switch" resulting in the xlator itself being turned on. We can use the newly introduced flags field in the volume_option_t struct.

[amarts]

I was thinking of solving it by approach mentioned in gluster/glusterfs#304 Can you check if that solves your problem?

[aravindavk]

With the templates support for Volgen(#1229). A Xlator can be enabled by setting [<graph-name>].<option-name>=on in Volinfo. (Volume Set/Reset need to be accommodated once template support merges)

Some Xlators understands enable by having a option in the graph as below,

volume gv7-changelog
    type features/changelog
    option changelog on
    subvolumes gv7-changetimerecorder
end-volume

And some of the Xlators doesn't need any option to say it is enabled. If xlator present in volfile means enabled. This can be handled by setting EnabledByOption field in Volfile template.

[aravindavk]

With Glusterfs Patch https://review.gluster.org/21813 and Glusterd2 patch #1370 we can enable/disable xlators if option name with same name as xlator exists in respective options table.