Ceph » rbd

The code would be in src/common.cc. I don't think it would be as simple as just adding another variable, because all of the existing variables are one-per-process, and a process can have multiple images open. The metavariable code would have to be made context-aware.

It's per-cct. We can probably add some methods like

cct->set_meta("image", "foo");

that would populate a map<string,string> and rerun the function that does the substitutions. I think part of the challenge will be making that happen early in the librbd setup, before the admin socket file is set up (probably before common_init() is called?). Should be doable, I think...

...which is hard because librbd layers on top of an existing librados ioctx, which has already long been initialized. We can't even introduce our own default values for anything.

Perhaps the thing to do is make the asok init fail/skip creating the file is there is a metavariable still present (i.e., $ appears in the string) and make the librbd explicit fill it in and then re-trigger something on the CephContext? Something like reopen_logs(), but slightly more general to also include the asok file.

This assumes that each image has its own cct, but a process could have multiple images open in one cct. (In fact, copy2 requires two open images.) Would we say that we don't support the image metavariable in that case?

What if an image is never opened? You'd never have a .asok.

Should the snap name be included? If so, should the metavariable have a different name?

I'd also like to point out that qemu opens and closes images twice, for some reason, just to make things more complicated.

It's not perfect, but we could add a cctid variable so users could specify something like "admin socket = /var/run/ceph/rbd-$pid-$cctid.asok". This ensures at least that the ccts don't stomp on each others' files, although you'd still have to manually figure out which .asok is for which image.

Yeah, that is probably a good idea anyway.. we've had uniqueness issues like this before! That is an easy thing and solves most of the problem

Glad to see this feature added. Thank you! Mark, Adam, Sage, and Loic!

Assuming it wouldn't be too difficult, could this be tagged to be backported to Dumpling as well as giant,firefly?