I was frequently getting Segmentation fault (core dumped). Not knowing what to do, I just restarted my ubuntu. Then tried repair which failed because of Segmentation fault. Now when I am trying to start mongod

root@ubuntu-s-4vcpu-8gb-nyc1-01:~/mongo# mongod --dbpath /var/lib/mongodb
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten] MongoDB starting : pid=2091 port=27017 dbpath=/var/lib/mongodb 64-bit host=ubuntu-s-4vcpu-8gb-nyc1-01
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten] db version v3.2.21
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten] git version: 1ab1010737145ba3761318508ff65ba74dfe8155
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten] OpenSSL version: OpenSSL 1.0.2g  1 Mar 2016
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten] allocator: tcmalloc
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten] modules: none
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten] build environment:
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten]     distmod: ubuntu1604
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten]     distarch: x86_64
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten]     target_arch: x86_64
2018-11-12T10:13:15.235+0000 I CONTROL  [initandlisten] options: { storage: { dbPath: "/var/lib/mongodb" } }
2018-11-12T10:13:15.255+0000 I -        [initandlisten] Detected data files in /var/lib/mongodb created by the 'wiredTiger' storage engine, so setting the active storage engine to 'wiredTiger'.
2018-11-12T10:13:15.255+0000 W -        [initandlisten] Detected unclean shutdown - /var/lib/mongodb/mongod.lock is not empty.
2018-11-12T10:13:15.255+0000 W STORAGE  [initandlisten] Recovering data from the last clean checkpoint.
2018-11-12T10:13:15.255+0000 I STORAGE  [initandlisten] wiredtiger_open config: create,cache_size=4G,session_max=20000,eviction=(threads_min=4,threads_max=4),config_base=false,statistics=(fast),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000),checkpoint=(wait=60,log_size=2GB),statistics_log=(wait=0),verbose=(recovery_progress),
2018-11-12T10:13:15.310+0000 I STORAGE  [initandlisten] dropping unused ident: index-0-8381493109628483945
2018-11-12T10:13:15.312+0000 I CONTROL  [initandlisten] ** WARNING: You are running this process as the root user, which is not recommended.
2018-11-12T10:13:15.312+0000 I CONTROL  [initandlisten] 
2018-11-12T10:13:15.312+0000 I CONTROL  [initandlisten] 
2018-11-12T10:13:15.312+0000 I CONTROL  [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/defrag is 'always'.
2018-11-12T10:13:15.312+0000 I CONTROL  [initandlisten] **        We suggest setting it to 'never'
2018-11-12T10:13:15.312+0000 I CONTROL  [initandlisten] 
2018-11-12T10:13:15.312+0000 I -        [initandlisten] Fatal assertion 28579 UnsupportedFormat: Unable to find metadata for table:index-0--3161084524998267994 Index: {name: _id_, ns: local.startup_log} - version too new for this mongod. See http://dochub.mongodb.org/core/3.4-index-downgrade for detailed instructions on how to handle this error.
2018-11-12T10:13:15.313+0000 I -        [initandlisten] 

***aborting after fassert() failure
mongod -vvvvvv -dbpath /var/lib/mongodb

and remove corrupted datafile.

rm f index-1234-123456789123456789.wt

and then repair using following command

mongod --repair --dbpath /var/lib/mongodb/ --storageEngine wiredTiger

Note: Have a backup of /var/lib/mongodb

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