The root cause is that the Cassandra process has run into system-imposed limits on the maximum number of open files. See the following code snippet for example of the error message:

Caused by: java.lang.RuntimeException: java.nio.file.FileSystemException: 
/path/data/system/compaction_history-b4dbb7b4dc493fb5b3bfce6e434832ca/
mc_txn_flush_8bdc78f0-7d48-11e9-9b2e-0f78ea2b6c2b.log: Too many open files

1. For Linux, enter the following commands in the Unix shell to check the limits on the number of open files:
   • To check the hard limit, enter ulimit -Hn

     Only the root user can raise this limit but any process can lower it.

   • To check the soft limit, enter ulimit -Sn

     Any process can change this limit.

2. Change the limit on the maximum number of open files, depending on your business needs.
   Do not raise the limit on open files above 100,000. For more information about changing open file limits, see the Apache Cassandra documentation.