I’m running a production 0.94.7 Ceph cluster, and have been seeing a periodic 
issue arise where in all my MDS clients will become stuck, and the fix so far 
has been to restart the active MDS (sometimes I need to restart the subsequent 
active MDS as well).

These clients are using the cephfs-hadoop API, so there is no kernel client, or 
fuse api involved. When I see clients get stuck, there are messages printed to 
stderr like the following:

2016-09-21 10:31:12.285030 7fea4c7fb700  0 – >> pipe(0x7feaa0a1e0f0 sd=206 :0 s=1 pgs=0 cs=0 l=0 

I’m at somewhat of a loss on where to begin debugging this issue, and wanted to 
ping the list for ideas.

I managed to dump the mds cache during one of the stalled moments, which 
hopefully is a useful starting point:

mdscachedump.txt.gz (https://filetea.me/t1sz3XPHxEVThOk8tvVTK5Bsg)


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