CephFS mirror very slow (maybe for small files?)

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Hi all.
I have successfully configured an operational mirror between 2 sites for Ceph FS. The mirroring is running but the speed of data transfer is varying a lot over time (200KB/s – 120MB/s). The network infrastructure between the two Ceph clusters is reliable and should not be the cause of this speed variation. The FS in question, has a lot of small files in it and I suspect this is the cause of the variability – ie, the transfer of many small files will be more impacted by greater site-site latency.
If this suspicion is true, what options do I have to improve the overall throughput?

  *   Is it possible to parallelise or “chunk” the transfers with some options to the mirror daemon?
  *   Would the use of multiple snapshot mirror points help?
     *   Note that I am currently forced to use only a single point ( ceph fs snapshot mirror add <fs_name> <path> ) because the part of the FS in use is managed by Openstack Manila which created a subvolume. Requests to add mirrors for sub-directories are therefore denied.
Any suggestions for how I can improve this throughput would be most welcome.
Currrently I am running Pacific (16.2.10) on the sender and Quincy (17.2.6) on the target.

Stuart Cornell
Cloud Development Director

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