On Fri, Feb 2, 2018 at 8:13 AM, Caspar Smit <caspars...@supernas.eu> wrote:
> Hi all,
>
> I'd like to setup a small cluster (5 nodes) using erasure coding. I would
> like to use k=5 and m=3.
> Normally you would need a minimum of 8 nodes (preferably 9 or more) for
> this.
>
> Then i found this blog:
> https://ceph.com/planet/erasure-code-on-small-clusters/
>
> This sounded ideal to me so i started building a test setup using the 5+3
> profile
>
> Changed the erasure ruleset to:
>
> rule erasure_ruleset {
>   ruleset X
>   type erasure
>   min_size 8
>   max_size 8
>   step take default
>   step choose indep 4 type host
>   step choose indep 2 type osd
>   step emit
> }
>
> Created a pool and now every PG has 8 shards in 4 hosts with 2 shards each,
> perfect.
>
> But then i tested a node failure, no problem again, all PG's stay active
> (most undersized+degraded, but still active). Then after 10 minutes the
> OSD's on the failed node were all marked as out, as expected.
>
> I waited for the data to be recovered to the other (fifth) node but that
> doesn't happen, there is no recovery whatsoever.
>
> Only when i completely remove the down+out OSD's from the cluster the data
> is recovered.
>
> My guess is that the "step choose indep 4 type host" chooses 4 hosts
> beforehand to store data on.

Hmm, basically, yes. The basic process is:

>   step take default

take the default root.

>   step choose indep 4 type host

Choose four hosts that exist under the root. *Note that at this layer,
it has no idea what OSDs exist under the hosts.*

>   step choose indep 2 type osd

Within the host chosen above, choose two OSDs.


Marking out an OSD does not change the weight of its host, because
that causes massive data movement across the whole cluster on a single
disk failure. The "chooseleaf" commands deal with this (because if
they fail to pick an OSD within the host, they will back out and go
for a different host), but that doesn't work when you're doing
independent "choose" steps.

I don't remember the implementation details well enough to be sure,
but you *might* be able to do something like

step take default
step chooseleaf indep 4 type host
step take default
step chooseleaf indep 4 type host
step emit

And that will make sure you get at least 4 OSDs involved?
-Greg

>
> Would it be possible to do something like this:
>
> Create a 5+3 EC profile, every hosts has a maximum of 2 shards (so 4 hosts
> are needed), in case of node failure -> recover data from failed node to
> fifth node.
>
> Thank you in advance,
> Caspar
>
>
>
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