Re: Failing heartbeats when no backfill is running

Lorenz Kiefner <root+cephusers@xxxxxxxxxxxx> · Thu, 15 Aug 2019 09:35:56 +0200



    Oh no, it's not that bad. It's 

    
    $ ping -s 65000 dest.inati.on
    on a VPN connection that has a MTU of 1300 via IPv6. So I suspect
      that I only get an answer, when all 51 fragments get fully
      returned. It's clear that big packets with lots of fragments are
      more affected by packet loss than 64 byte pings.

    
    I just (at 9 o'clock in the morning) repeated this ping test and
      got hardly any drops (less than 1%), even with the size of 64k. So
      it's really dependent on the time of the day. Seems like some ISPs
      are dropping some packets, especially in the evening...

    
    A few minutes ago I restarted all down-marked OSDs, but they are
      getting marked down again... Seems like Ceph is tolerable against
      packet loss (it surely affects performance, but this irrelevant
      for me).
    

    Could erasure coded pools pose some problems?

    
    Thank you all for every hint!
    Lorenz

    
    Am 15.08.19 um 08:51 schrieb Janne
      Johansson:

    
        Den ons 14 aug. 2019 kl 17:46 skrev Lorenz
          Kiefner <root+cephusers@xxxxxxxxxxxx>:

        
          Is ceph sensitive to
            packet loss? On some VPN links I have up to 20%

            packet loss on 64k packets but less than 3% on 5k packets in
            the evenings.

          
          20% seems crazy high, there must be something really
            wrong there.
          

          At 20%, you would get tons of packet timeouts to wait for
            on all those lost frames,
          then resends of (at least!) those 20% extra, which in
            turn would lead to 20% of those
          resends getting lost, all while the main streams of data
            try to move forward when some
          older packet do get over. This is a really bad situation
            to design for, 
          

          I think you should look for a link solution that doesn't
            drop that many packets, instead of changing
          the software you try to run over that link, all others
            will notice this too and act badly in some way or other.
          

          Heck, 20% is like taking a math schoolbook and remove all
            instances of "3" and "8" and see if kids can learn to count
            from it. 8-/
           
        
        -- 

        May the most significant
          bit of your life be positive.

        
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