Docs: note about snapshot version compatibility (elastic#20896)

nik9000 · nik9000 · commit 314c2a0c5fdd · 2016-10-13T11:14:28.000-04:00
It is important that folks understand that snapshot/restore isn't for archiving. It is appropriate for backup and disaster recovery but not for archival over long periods of time because of version incompatibility. Closes elastic#20866
diff --git a/docs/reference/modules/snapshots.asciidoc b/docs/reference/modules/snapshots.asciidoc
@@ -1,9 +1,20 @@
 [[modules-snapshots]]
 == Snapshot And Restore
 
-The snapshot and restore module allows to create snapshots of individual indices or an entire cluster into a remote
-repository. At the time of the initial release only shared file system repository was supported, but now a range of
-backends are available via officially supported repository plugins.
+The snapshot and restore module allows to create snapshots of individual
+indices or an entire cluster into a remote repository like shared file system,
+S3, or HDFS. These snapshots are great for backups because they can be restored
+relatively quickly but they are not archival because they can only be restored
+to versions of Elasticsearch that can read the index. That means that:
+
+* A snapshot of an index created in 2.x can be restored to 5.x.
+* A snapshot of an index created in 1.x can be restored to 2.x.
+* A snapshot of an index created in 1.x can **not** be restored to 5.x.
+
+To restore a snapshot of an index created in 1.x to 5.x you can restore it to
+a 2.x cluster and use reindex-from-remote (only available in 5.0+) to rebuild
+the index in a 5.x cluster. This is as time consuming as restoring from
+archival copies of the original data.
 
 [float]
 === Repositories
@@ -474,5 +485,3 @@ that snapshot before deleting the snapshot data from the repository.
 The restore operation uses the standard shard recovery mechanism. Therefore, any currently running restore operation can
 be canceled by deleting indices that are being restored. Please note that data for all deleted indices will be removed
 from the cluster as a result of this operation.
-
-
