Re: How to tell a VM to write more local ceph nodes than to the network.

Lionel Bouton <lionel+ceph@xxxxxxxxxxx> · Wed, 14 Jan 2015 16:27:56 +0100



    On 01/13/15 22:03, Roland Giesler
      wrote:

    
        I
          have a 4 node ceph cluster, but the disks are not equally
          distributed across all machines (they are substantially
          different from each other)

          
        One
          machine has 12 x 1TB SAS drives (h1), another has 8 x 300GB
          SAS (s3) and two machines have only two 1 TB drives each (s2
          & s1).

          
        Now
          machine s3 has by far the most CPU's and RAM, so I'm running
          my VM's mostly from there, but I want to make sure that the
          writes that happen to the ceph cluster get written to the
          "local" osd's on s3 first and then the additional
          writes/copies get done to the network.

          
        Is
          this possible with ceph.  The VM's are KVM in Proxmox in case
          it's relevant.

        
    I don't think it is possible because I believe it would break its
    durability guarantees: IIRC each write is only considered done when
    all replicas have been written to (with default settings 3 replicas
    on 3 different servers) so you have to wait for 3 servers to
    acknowledge the write for it to complete in the VM.

    
    You could maybe achieve what you want by using tiering with a cache
    pool configured to use only disks on the s3 server in write-back
    mode but given the user experience reports on the list it may
    actually perform worse than your current setup.

    
    Best regards,

    
    Lionel Bouton

  
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