On Mon, Jul 16, 2018 at 1:25 AM John Spray <jsp...@redhat.com> wrote:

> On Sun, Jul 15, 2018 at 12:46 PM Oliver Schulz
> <oliver.sch...@tu-dortmund.de> wrote:
> >
> > Dear all,
> >
> > we're planning a new Ceph-Clusterm, with CephFS as the
> > main workload, and would like to use erasure coding to
> > use the disks more efficiently. Access pattern will
> > probably be more read- than write-heavy, on average.
> >
> > I don't have any practical experience with erasure-
> > coded pools so far.
> >
> > I'd be glad for any hints / recommendations regarding
> > these questions:
> >
> > * Is an SSD cache pool recommended/necessary for
> >    CephFS on an erasure-coded HDD pool (using Ceph
> >    Luminous and BlueStore)?
>
> Since Luminous, you can use an erasure coded pool (on bluestore)
> directly as a CephFS data pool, no cache pool needed.
>

More than that, we'd really prefer you didn't use cache pools for anything.
Just Say No. :)
-Greg


>
> John
>
> > * What are good values for k/m for erasure coding in
> >    practice (assuming a cluster of about 300 OSDs), to
> >    make things robust and ease maintenance (ability to
> >    take a few nodes down)? Is k/m = 6/3 a good choice?
>

That will depend on your file sizes, IO patterns, and expected durability
needs. I think 6+3 is a common one but I don't deal with many deployments.


> >
> > * Will it be sufficient to have k+m racks, resp. failure
> >    domains?
>

Generally, if you want CRUSH to select X "buckets" at any level, it's good
to have at least X+1 choices for it to prevent mapping failures. But you
could also do workaround like letting it choose (K+M)/2 racks and putting
two shards in each rack.
-Greg


> >
> >
> > Cheers and thanks for any advice,
> >
> > Oliver
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