Today we fail the node at startup if it contains an index that is too old to be
compatible with the current version, unless that index is closed. If the index
is closed then the node will start up and this puts us into a bad state: the
index cannot be opened and must be reindexed using an earlier version, but we
offer no way to get that index into a node running an earlier version so that
it can be reindexed. Downgrading the node in-place is decidedly unsupported and
cannot be expected to work since the node already started up and upgraded the
rest of its metadata. Since #41731 we actively reject downgrades to versions ≥
v7.2.0 too.
This commit prevents the node from starting in the presence of any too-old
indices (closed or not). In particular, it does not write any upgraded metadata
in this situation, increasing the chances an in-place downgrade might be
successful. We still actively reject the downgrade using #41731, because we
wrote the node metadata file before checking the index metadata, but at least
there is a way to override this check.
Relates #21830, #44230
With the removal of node.max_local_storage_nodes, there is no need anymore to keep the data in
subfolders indexed by a node ordinal. This commit makes it so that ES 8.0 will store data directly in
$DATA_DIR instead of $DATA_DIR/nodes/$nodeOrdinal.
Upon startup, Elasticsearch will check to see if there is data in the old location, and automatically
move it to the new location. This automatic migration only works if $nodeOrdinal is 0, i.e., multiple
node instances have not previously run on the same data path, which required for
node.max_local_storage_nodes to explicitly be configured.
This setting, which prior to Elasticsearch 5 was enabled by default and caused all kinds of
confusion, has since been disabled by default and is not recommended for production use. The
preferred way going forward is for users to explicitly specify separate data folders for each started
node to ensure that each node is consistently assigned to the same data path.
Relates to #42426
debadair
James Rodewig
James Rodewig
David Turner
Yannick Welsch