During upgrades from 7.x to 8.x if a node was holding a frozen cache file, the system considers that as an unknown file and prevents starting the node:
```
java.lang.IllegalStateException: unexpected file/folder encountered during data folder upgrade: /app/data/nodes/0/shared_snapshot_cache
	at org.elasticsearch.env.NodeEnvironment.upgradeLegacyNodeFolders(NodeEnvironment.java:443) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.env.NodeEnvironment.<init>(NodeEnvironment.java:290) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.node.Node.<init>(Node.java:388) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.node.Node.<init>(Node.java:277) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Bootstrap$5.<init>(Bootstrap.java:234) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Bootstrap.setup(Bootstrap.java:234) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Bootstrap.init(Bootstrap.java:358) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.init(Elasticsearch.java:166) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.execute(Elasticsearch.java:157) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.cli.EnvironmentAwareCommand.execute(EnvironmentAwareCommand.java:77) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.cli.Command.mainWithoutErrorHandling(Command.java:112) [elasticsearch-cli-8.0.1.jar:8.0.1]
	at org.elasticsearch.cli.Command.main(Command.java:77) [elasticsearch-cli-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:122) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:80) [elasticsearch-8.0.1.jar:8.0.1]
```
These files are expected files and should be ignored/accepted during the data folder upgrade.
server/src/main/java/org/elasticsearch/env/NodeEnvironment.java
} else {
            throw new IllegalArgumentException("no custom " + IndexMetadata.SETTING_DATA_PATH + " setting available");
        }
    }
    /**
     * Resolve the custom path for a index's shard.
     * Uses the {@code IndexMetadata.SETTING_DATA_PATH} setting to determine
     * the root path for the index.
     *
     * @param customDataPath the custom data path
     */
    private Path resolveIndexCustomLocation(String customDataPath, String indexUUID) {
        return resolveIndexCustomLocation(customDataPath, indexUUID, sharedDataPath);
    }
    private static Path resolveIndexCustomLocation(String customDataPath, String indexUUID, Path sharedDataPath) {
        return resolveBaseCustomLocation(customDataPath, sharedDataPath).resolve(indexUUID);
    }
    /**
     * Resolve the custom path for a index's shard.
     * Uses the {@code IndexMetadata.SETTING_DATA_PATH} setting to determine
     * the root path for the index.
     *
     * @param customDataPath the custom data path
     * @param shardId shard to resolve the path to
     */
    public Path resolveCustomLocation(String customDataPath, final ShardId shardId) {
        return resolveCustomLocation(customDataPath, shardId, sharedDataPath);
    }
    public static Path resolveCustomLocation(String customDataPath, final ShardId shardId, Path sharedDataPath) {
        return resolveIndexCustomLocation(customDataPath, shardId.getIndex().getUUID(), sharedDataPath).resolve(