Hi,

You will get slow performance: EC is slow, HDD are slow too

With 400 iops per device, you get 89600 iops for the whole cluster, raw
With 8+3EC, each logical write is mapped to 11 physical writes
You get only 8145 write IOPS (is my math correct ?), which I find very
low for a PB storage

So, use flash-based storage, or at least not EC (which is for cold storage)

>  * We're used to run mons and mgrs daemons on a few of our OSD nodes, without
>    any issue so far : is this a bad idea for a big cluster ?
Nothing troublesome here

>  * We thought using cache tiering on an SSD pool, but a large part of the PB 
> is
>    used on a daily basis, so we expect the cache to be not so effective and
>    really expensive ?
Cache tiering is not recommended
You will not be able to manage this cache (managing which content needs
to be cached), thus degrading performances

>  * Could a 2x10G network be enough ?
If you buy hardware, get 25Gbps

> 
> Thanks for reading, and sharing your experiences !
> 
> F.
> 
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