It's OneFS FEC protection nomenclature for our "hybrid" protection polices.  More generally, it's written as:  N+M/b  where "N" is the number of data stripe units in a stripe, "M" is the number of FEC stripe units in a stripe, and "b" is the number of stripe units in a stripe which are allowed to reside on the same node.  For the hybrid protection policy of +2:1 (or +2d:1n in OneFS 7.2+), you have  N+2/2  where "N" varies, up to our maximum stripe width, based on cluster size.  For a 3 node cluster, N=4.  So your 4+2/2 says you have a 3 node cluster, and each stripe of a file can have (at most) 4 data stripe units, 2 FEC stripe units, with at most 2 stripe units, per stripe, per node.  This protection policy can sustain the failure of a single node or the failure of 2 drives.
Hi guys  Do you know what this /2 meant by The out put from #isi get cmd?   Isi get output summary    /ifs/home/newUser1: default 4+2/2 concurrency on .cshrc   I know this is protection n+2:1 , but not sure /2 for. Would this be up to 2 disks can be failed? Or this mean divided by 2?   Thanks!
Isi get output