Add an option to fix the pathological space usage of RocksDB on btrfs.

tuwunel-example.toml

@@ -1142,6 +1142,18 @@
 #
 #rocksdb_never_drop_columns = false
 
+# Configures RocksDB to not preallocate WAL logs.
+#
+# Normally, RocksDB allocates certain types of files by calling
+# fallocate, writing the file contents, then truncating the logs to the
+# proper size. This causes pathological disk space usage on btrfs due
+# how it interacts with its Copy-on-Write implementation.
+#
+# It is recommended to set this to false if you run the server on btrfs,
+# and not touch it otherwise.
+#
+#rocksdb_allow_fallocate = true
+
 # This is a password that can be configured that will let you login to the
 # server bot account (currently `@conduit`) for emergency troubleshooting
 # purposes such as recovering/recreating your admin room, or inviting