I have 22 shards cluster running for 2 years.

Without any reason, one of shards was missing host:port info, which caused application read faliures.

Can you see if it's known issue ?

Mongo is running on V2.6.12.

mongos> db.shards.find();

{ "_id" : "TT-RS01", "host" : "TT-RS01/tdb-a01.white.xxx.com:4308,tdb-m01.white.xxx.com:4308" } { "_id" : "TT-RS02", "host" : "TT-RS02/tdb-a02.white.xxx.com:4308,tdb-m02.white.xxx.com:4308" } { "_id" : "TT-RS03", "host" : "TT-RS03/tdb-a03.white.xxx.com:4308,tdb-m03.white.xxx.com:4308" } { "_id" : "TT-RS05", "host" : "TT-RS05/tdb-a05.white.xxx.com:4308,tdb-m05.white.xxx.com:4308" } { "_id" : "TT-RS06", "host" : "TT-RS06/tdb-a06.white.xxx.com:4308,tdb-m06.white.xxx.com:4308" } { "_id" : "TT-RS04", "host" : "TT-RS04/tdb-a04.white.xxx.com:4308,tdb-m04.white.xxx.com:4308" } { "_id" : "TT-RS07", "host" : "TT-RS07/" } { "_id" : "TT-RS08", "host" : "TT-RS08/tdb-a08.white.xxx.com:4308,tdb-m08.white.xxx.com:4308" } { "_id" : "TT-RS09", "host" : "TT-RS09/tdb-a09.white.xxx.com:4308,tdb-m09.white.xxx.com:4308" } { "_id" : "TT-RS10", "host" : "TT-RS10/tdb-a10.white.xxx.com:4308,tdb-m10.white.xxx.com:4308" } { "_id" : "TT-RS11", "host" : "TT-RS11/tdb-a11.white.xxx.com:4308,tdb-m11.white.xxx.com:4308" } { "_id" : "TT-RS12", "host" : "TT-RS12/tdb-a12.white.xxx.com:4308,tdb-m12.white.xxx.com:4308" } { "_id" : "TT-RS13", "host" : "TT-RS13/tdb-a13.white.xxx.com:4308,tdb-m13.white.xxx.com:4308" } { "_id" : "TT-RS14", "host" : "TT-RS14/tdb-a14.white.xxx.com:4308,tdb-m14.white.xxx.com:4308" } { "_id" : "TT-RS15", "host" : "TT-RS15/tdb-a15.white.xxx.com:4308,tdb-m15.white.xxx.com:4308" } { "_id" : "TT-RS16", "host" : "TT-RS16/tdb-a16.white.xxx.com:4308,tdb-m16.white.xxx.com:4308" } { "_id" : "TT-RS17", "host" : "TT-RS17/tdb-a17.white.xxx.com:4308,tdb-m17.white.xxx.com:4308" } { "_id" : "TT-RS18", "host" : "TT-RS18/tdb-a18.white.xxx.com:4308,tdb-m18.white.xxx.com:4308" } { "_id" : "TT-RS19", "host" : "TT-RS19/tdb-a19.white.xxx.com:4308,tdb-m19.white.xxx.com:4308" } { "_id" : "TT-RS20", "host" : "TT-RS20/tdb-a20.white.xxx.com:4308,tdb-m20.white.xxx.com:4308" }

Type "it" for more

I did manually update shard07 entry.
db.shards.update(

{ "_id" : "TT-RS07"}

,

{"_id": "TT-RS07", "host": "TT-RS07/tdb-a07.white.xxx.com:4308,tdb-m07.white.xxx.com:4308"}

);
mongos> db.shards.find();

{ "_id" : "TT-RS01", "host" : "TT-RS01/tdb-a01.white.xxx.com:4308,tdb-m01.white.xxx.com:4308" } { "_id" : "TT-RS02", "host" : "TT-RS02/tdb-a02.white.xxx.com:4308,tdb-m02.white.xxx.com:4308" } { "_id" : "TT-RS03", "host" : "TT-RS03/tdb-a03.white.xxx.com:4308,tdb-m03.white.xxx.com:4308" } { "_id" : "TT-RS05", "host" : "TT-RS05/tdb-a05.white.xxx.com:4308,tdb-m05.white.xxx.com:4308" } { "_id" : "TT-RS06", "host" : "TT-RS06/tdb-a06.white.xxx.com:4308,tdb-m06.white.xxx.com:4308" } { "_id" : "TT-RS04", "host" : "TT-RS04/tdb-a04.white.xxx.com:4308,tdb-m04.white.xxx.com:4308" } { "_id" : "TT-RS07", "host" : "TT-RS07/tdb-a07.white.xxx.com:4308,tdb-m07.white.xxx.com:4308" } { "_id" : "TT-RS08", "host" : "TT-RS08/tdb-a08.white.xxx.com:4308,tdb-m08.white.xxx.com:4308" } { "_id" : "TT-RS09", "host" : "TT-RS09/tdb-a09.white.xxx.com:4308,tdb-m09.white.xxx.com:4308" } { "_id" : "TT-RS10", "host" : "TT-RS10/tdb-a10.white.xxx.com:4308,tdb-m10.white.xxx.com:4308" } { "_id" : "TT-RS11", "host" : "TT-RS11/tdb-a11.white.xxx.com:4308,tdb-m11.white.xxx.com:4308" } { "_id" : "TT-RS12", "host" : "TT-RS12/tdb-a12.white.xxx.com:4308,tdb-m12.white.xxx.com:4308" } { "_id" : "TT-RS13", "host" : "TT-RS13/tdb-a13.white.xxx.com:4308,tdb-m13.white.xxx.com:4308" } { "_id" : "TT-RS14", "host" : "TT-RS14/tdb-a14.white.xxx.com:4308,tdb-m14.white.xxx.com:4308" } { "_id" : "TT-RS15", "host" : "TT-RS15/tdb-a15.white.xxx.com:4308,tdb-m15.white.xxx.com:4308" } { "_id" : "TT-RS16", "host" : "TT-RS16/tdb-a16.white.xxx.com:4308,tdb-m16.white.xxx.com:4308" } { "_id" : "TT-RS17", "host" : "TT-RS17/tdb-a17.white.xxx.com:4308,tdb-m17.white.xxx.com:4308" } { "_id" : "TT-RS18", "host" : "TT-RS18/tdb-a18.white.xxx.com:4308,tdb-m18.white.xxx.com:4308" } { "_id" : "TT-RS19", "host" : "TT-RS19/tdb-a19.white.xxx.com:4308,tdb-m19.white.xxx.com:4308" } { "_id" : "TT-RS20", "host" : "TT-RS20/tdb-a20.white.xxx.com:4308,tdb-m20.white.xxx.com:4308" }

Type "it" for more