--- /dev/null
+==================
+D3N RGW Data Cache
+==================
+
+.. contents::
+
+Datacenter-Data-Delivery Network (D3N) uses high-speed storage such as NVMe flash or DRAM to cache
+datasets on the access side.
+Such caching allows big data jobs to use the compute and fast storage resources available on each
+Rados Gateway node at the edge.
+
+Many datacenters include low-cost, centralized storage repositories, called data lakes,
+to store and share terabyte and petabyte-scale datasets.
+By necessity most distributed big-data analytic clusters such as Hadoop and Spark must
+depend on accessing a centrally located data lake that is relatively far away.
+Even with a well-designed datacenter network, cluster-to-data lake bandwidth is typically much less
+than the bandwidth of a solid-state storage located at an edge node.
+
+| D3N improves the performance of big-data jobs running in analysis clusters by speeding up recurring reads from the data lake.
+| The Rados Gateways act as cache servers for the back-end object store (OSDs), storing data locally for reuse.
+
+Architecture
+============
+
+D3N improves the performance of big-data jobs by speeding up repeatedly accessed dataset reads from the data lake.
+Cache servers are located in the datacenter on the access side of potential network and storage bottlenecks.
+D3Ns two-layer logical cache forms a traditional caching hierarchy :sup:`*`
+where caches nearer the client have the lowest access latency and overhead,
+while caches in higher levels in the hierarchy are slower (requiring multiple hops to access),
+The layer 1 cache server nearest to the client handles object requests by breaking them into blocks,
+returning any blocks which are cached locally, and forwarding missed requests to the block home location
+(as determined by consistent hashing) in the next layer.
+Cache misses are forwarded to successive logical caching layers until a miss at the top layer is resolved
+by a request to the data lake (Rados)
+
+:sup:`*` currently only layer 1 cache has been upstreamed.
+
+See `MOC D3N (Datacenter-scale Data Delivery Network)`_ and `Red Hat Research D3N Cache for Data Centers`_.
+
+Implementation
+==============
+
+- The D3N cache supports both the `S3` and `Swift` object storage interfaces.
+- D3N currently caches only tail objects, because they are immutable (by default it is parts of objects that are larger than 4MB).
+ (the NGINX `RGW Data cache and CDN`_ supports caching of all object sizes)
+
+
+Requirements
+------------
+
+- An SSD (/dev/nvme,/dev/pmem,/dev/shm) or similar block storage device, formatted
+ (filesystems other than XFS were not tested) and mounted.
+ It will be used as the cache backing store.
+ (depending on device performance, multiple RGWs may share a single device but each requires
+ a discrete directory on the device filesystem)
+
+Limitations
+-----------
+
+- D3N will not cache objects compressed by `Rados Gateway Compression`_ (OSD level compression is supported).
+- D3N will not cache objects encrypted by `Rados Gateway Encryption`_.
+- D3N will be disabled if the ``rgw_max_chunk_size`` config variable value differs from the ``rgw_obj_stripe_size`` config variable value.
+
+
+D3N Environment Setup
+=====================
+
+Running
+-------
+
+To enable D3N on an existing RGWs the following configuration entries are required
+in each Rados Gateways ceph.conf client section, for example for ``[client.rgw.8000]``::
+
+ [client.rgw.8000]
+ rgw_d3n_l1_local_datacache_enabled = true
+ rgw_d3n_l1_datacache_persistent_path = "/mnt/nvme0/rgw_datacache/client.rgw.8000/"
+ rgw_d3n_l1_datacache_size = 10737418240
+
+The above example assumes that the cache backing-store solid state device
+is mounted at `/mnt/nvme0` and has `10 GB` of free space available for the cache.
+
+The persistent path directory has to be created before starting the Gateway.
+(``mkdir -p /mnt/nvme0/rgw_datacache/client.rgw.8000/``)
+
+If another Gateway is co-located on the same machine, configure it's persistent path to a discrete directory,
+for example in the case of `[client.rgw.8001]` configure
+``rgw_d3n_l1_datacache_persistent_path = "/mnt/nvme0/rgw_datacache/client.rgw.8001/"``
+in the ``[client.rgw.8001]`` ceph.conf client section.
+
+In a multiple co-located Gateways configuration consider assigning clients with different workloads
+to each Gateway without a balancer in order to avoid cached data duplication.
+
+ NOTE: each time the Rados Gateway is restarted the content of the cache directory is purged.
+
+Logs
+----
+- D3N related log lines in `radosgw.*.log` contain the string ``d3n`` (case insensitive).
+- low level D3N logs can be enabled by the ``debug_rgw_datacache`` subsystem (up to ``debug_rgw_datacache=30``)
+
+
+CONFIG REFERENCE
+================
+The following D3N related settings can be added to the Ceph configuration file
+(i.e., usually `ceph.conf`) under the ``[client.rgw.{instance-name}]`` section.
+
+.. confval:: rgw_d3n_l1_local_datacache_enabled
+.. confval:: rgw_d3n_l1_datacache_persistent_path
+.. confval:: rgw_d3n_l1_datacache_size
+.. confval:: rgw_d3n_l1_eviction_policy
+
+
+.. _MOC D3N (Datacenter-scale Data Delivery Network): https://massopen.cloud/research-and-development/cloud-research/d3n/
+.. _Red Hat Research D3N Cache for Data Centers: https://research.redhat.com/blog/research_project/d3n-multilayer-cache/
+.. _Rados Gateway Compression: ../compression/
+.. _Rados Gateway Encryption: ../encryption/
+.. _RGW Data cache and CDN: ../rgw-cache/
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