Ceph distributed over slow link: possible?

[Matthias Urlichs <matthias@urlichs.de>]

Hello ceph people,

My situation is this: my Ceph cluster is distributed over multiple
sites. The links between sites are rather slow. :-/

Storing one copy of a file at each site should not be a problem with
a reasonable crushmap, but ..:

* how can I verify on which devices a file is stored?

* is it possible to teach clients to read/write from "their", i.e.
  the local site's, copy of a file, instead of pulling stuff from
  a remote site? Or does ceph notice the speed difference by itself?

* My crushmap looks like this:
type 0  device
type 1  host
type 2  site
type 3  root
... (root => 2 sites => 2 hosts each => 3 devices each)
rule  data {
        ruleset 0
        type replicated
        min_size 2
        max_size 2
        step take  root
        step chooseleaf firstn 2 type site
        step emit
}

but when only one site is reachable, will there be one or two
copies of a file? If the former, how do I fix that? If the latter,
will the copy be redistributed when (the link to) the second site
comes back?

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