Re: issues with adjusting the crushmap in 0.51

[Jimmy Tang <jtang@tchpc.tcd.ie>]

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Hi Greg

On Thu, Sep 06, 2012 at 11:19:12AM -0700, Gregory Farnum wrote:
> You always need to end up with "devices" (the OSDs, generally) and
> then emit those from your CRUSH rule. You can do so hierarchically:
> rule data {
>         ruleset 0
>         type replicated
>         min_size 1
>         max_size 10
>         step take default
>         step choose firstn 0 type host
>         step choose firstn 1 osd
>         step emit
> }
> In this case, (with n being your replication count) this rule chooses
> n hosts, and then chooses 1 OSD from each chosen host.
> 
> You can also use "chooseleaf", which is a bit more robust in the
> presence of failed OSDs:
> rule data {
>         ruleset 0
>         type replicated
>         min_size 1
>         max_size 10
>         step take default
>         step chooseleaf firstn 0 type host
>         step emit
> }
> This rule will choose n hosts and an OSD from each chosen host, and if
> it fails on any host then it will restart with a different host (the
> previous rule would stick with the chosen hosts and so it can't handle
> if eg an entire host's OSDs are down).
> -Greg
> 

That explaination certainly has cleared things up for me, I had not
realised that I needed to end with a "device" at the end of a rule
based on the documentation that I could find on the website and the
old ceph wiki.

Also, the "ceph osd setcrushmap..." command doesn't up when a ceph
--help is run in the 0.51 release, however it is documented on the
wiki as far as I recall. It'd be real nice if the applications emitted
all the available commands, it would make experimenting much nicer and
fun.

Thanks,
Jimmy,

-- 
Jimmy Tang
Trinity Centre for High Performance Computing,
Lloyd Building, Trinity College Dublin, Dublin 2, Ireland.
http://www.tchpc.tcd.ie/

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