elasticsearch/docs/reference/migration/migrate_5_0/fs.asciidoc

[[breaking_50_fs]]
=== Filesystem related changes

Only a subset of index files were open with `mmap` on Elasticsearch 2.x. As of
Elasticsearch 5.0, all index files will be open with `mmap` on 64-bit systems.
While this may increase the amount of virtual memory used by Elasticsearch,
there is nothing to worry about since this is only address space consumption
and the actual memory usage of Elasticsearch will stay similar to what it was
in 2.x. See http://blog.thetaphi.de/2012/07/use-lucenes-mmapdirectory-on-64bit.html
for more information.

=== Path to data on disk

In prior versions of Elasticsearch, the `path.data` directory included a folder
for the cluster name, so that data was in a folder such as
`$DATA_DIR/$CLUSTER_NAME/nodes/$nodeOrdinal`. In 5.0 the cluster name as a
directory is deprecated. Data will now be stored in
`$DATA_DIR/nodes/$nodeOrdinal` if there is no existing data. Upon startup,
Elasticsearch will check to see if the cluster folder exists and has data, and
will read from it if necessary. In Elasticsearch 6.0 this backwards-compatible
behavior will be removed.

If you are using a multi-cluster setup with both instances of Elasticsearch
pointing to the same data path, you will need to add the cluster name to the
data path so that different clusters do not overwrite data.

==== Local files

Prior to 5.0, nodes that were marked with both `node.data: false` and `node.master: false` (or the now removed `node.client: true`)
didn't write any files or folder to disk. 5.x added persistent node ids, requiring nodes to store that information. As such, all
node types will write a small state file to their data folders.