Hi, Christian Balzer wrote :
> Consider what you think your IO load (writes) generated by your client(s) > will be, multiply that by your replication factor, divide by the number of > OSDs, that will give you the base load per OSD. > Then multiply by 2 (journal on OSD) per OSD. > Finally based on my experience and measurements (link below) multiply that > by at least 6, probably 10 to be on safe side. Use that number to find the > SSD that can handle this write load for the time period you're budgeting > that cluster for. > http://lists.opennebula.org/pipermail/ceph-users-ceph.com/2014-October/043949.html Thanks Christian for this interesting explanations. I have read your link and I'd like to understand why the write amplification is greater than the the replication factor. For me, in theory, write amplification should be approximatively equal to the replication factor. What are the reasons of this difference? Er... in fact, after thinking about it a little, I imagine that 1 write IO in the client side becomes 2*R IO in the cluster side (where R is the replication factor) because there are R IO for the OSD and R IO for the journal. So, with R = 2, I can imagine a write amplification equal to 4 but I don't understand why it's 5 or 6. Is it possible to have explanations about this? -- François Lafont _______________________________________________ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
