HADOOP-18328. Add documentation for S3A support on S3 Outposts (#5976)

Contributed by Yuting Chen

hadoop-tools/hadoop-aws/src/site/markdown/tools/hadoop-aws/index.md

@@ -1709,6 +1709,24 @@ the storage class you want.
 Please note that S3A does not support reading from archive storage classes at the moment.
 `AccessDeniedException` with InvalidObjectState will be thrown if you're trying to do so.
 
+## <a name="upload"></a>Configuring S3A for S3 on Outposts
+
+S3A now supports [S3 on Outposts](https://docs.aws.amazon.com/AmazonS3/latest/userguide/S3onOutposts.html).
+Accessing data through an access point is done by using its Amazon Resource Name (ARN), as opposed to just the bucket name.
+The only supported storage class on Outposts is **OUTPOSTS**, and by default objects are encrypted with [SSE-S3](https://docs.aws.amazon.com/AmazonS3/latest/userguide/s3-outposts-data-encryption.html).
+You can set the Access Point ARN property using the following per bucket configuration property:
+
+```xml
+<property>
+  <name>fs.s3a.bucket.sample-outpost-bucket.accesspoint.arn</name>
+  <value>arn:aws:s3-outposts:region:account-id:outpost/outpost-id/accesspoint/accesspoint-name</value>
+  <description>Configure S3a traffic to use this S3 on Outposts Access Point ARN</description>
+</property>
+```
+
+This configures access to the `sample-outpost-bucket` for S3A to go through the new Access Point ARN. So, for example `s3a://sample-outpost-bucket/key` will now use your configured ARN when getting data from S3 on Outpost instead of your bucket.
+
+
 ## <a name="upload"></a>How S3A writes data to S3
 
 The original S3A client implemented file writes by