Re: Activate Cache Tier on Running Pools

Wido den Hollander <wido@xxxxxxxx> · Mon, 16 Sep 2019 11:51:45 +0200



    On 9/16/19 11:36 AM, Eikermann, Robert
      wrote:

    
        Hi,
         
        I’m using Ceph in
            combination with Openstack. For the “VMs” Pool I’d like to
            enable writeback caching tier, like described here:
          https://docs.ceph.com/docs/luminous/rados/operations/cache-tiering/ .
         
      
    Can you explain why? The cache tiering has some serious flaws and
      can even decrease performance instead of improve it.
    What are you trying to solve?
    Wido
    
      
        Should it be possible to
            do that on a running pool? I tried to do so and immediately
            all VMs (Linux Ubuntu OS) running on Ceph disks got readonly
            filesystems. No errors were shown in ceph (but also no
            traffic arrived after enabling the cache tier). Removing the
            cache tier , rebooting the VMs and doing a filesystemcheck
            repaired everything.
         
        Best
        Robert
         
      
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