[GitHub] [ozone] elek commented on a change in pull request #1419: HDDS-3755. [DESIGN] Storage-class for Ozone

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elek commented on a change in pull request #1419:
URL: https://github.com/apache/ozone/pull/1419#discussion_r536085326



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File path: hadoop-hdds/docs/content/design/storage-class.md
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@@ -19,10 +20,408 @@ author: Marton Ele
   See the License for the specific language governing permissions and
   limitations under the License. See accompanying LICENSE file.
 -->
+
+
 # Abstract
 
-Proposal suggest to introduce a new storage-class abstraction which can be used to define different replication strategies (factor, type, ...) for different bucket/keys.
+One of the fundamental abstraction of Ozone is the _Container_ which used as the unit of the replication.
+
+Containers have to favors: _Open_ and _Closed_ containers: Open containers are replicated by Ratis and writable, Closed containers are replicated with simple data copy and they are read only.
+
+In this document a new level of abstraction is proposed: the *storage-class* which defines the replication mechanism of the containers and the parameters of the replication.
+
+# Goals / Use cases
+
+## [USER] Simplify user interface and improve usability
+
+Users can choose from an admin provided set of storage classes (for example *STANDARD*, *REDUCED*) instead of using implementation specific terms (*RATIS/THREE*, *RATIS/ONE*)
+
+Today the users should use implementation specific terms when key is created:
+
+```
+ozone sh key put --replication=THREE --type=RATIS /vol1/b1/key1 a.txt
+```
+
+There are two problems here:
+
+ 1. User should use low-level, technical terms during the usage. User might not know what is *RATIS* and may not have enough information to decide the right replication scheme.
+
+ 2. The current parameters defines only the replication of the *open* containers and the same value (THREE) is used for *Closed* containers. There is no easy way to add configuration which can be used later during the lifecycle of containers/keys. No easy way to adjust behavior of the replication. (For example to support `Ratis/THREE` --> `Ratis/TWO`)
+
+With the storage-class abstraction the complexity of configuration can be moved to the admin side, where we can provide more flexibility. Users should choose only from the available storage-classes (or use the default one) and the exact replication mechanism is defined by the administrator for each of the storage-.classes
+
+Instead of the current CLI parameters (replication factor and type) this document proposes to use an abstract storage-class parameter instead:
+
+```
+ozone sh key put --storage-class=STANDARD /vol1/bucket1/key1 source-file.txt
+```
+
+## [USER] Set a custom replication for a newly created bucket
+
+A user may want to set a custom replication for a bucket at the time of creation. All keys in the bucket will respect the specified storage-class (subject to storage and quota availability). E.g.
+
+```
+ozone sh bucket create --storage-class=INFREQUENT_ACCESS
+```
+
+
+Bucket-level default storage-class can be overridden for any key, but will be used as default.
+
+
+## [USER] Fine grained replication control when using S3 API
+
+A user may want to set custom replication policies for any key **which uploaded via S3 API**. Storage-classes are already used by AWS S3 API. With first-class support of the same concept in Ozone, users can choose from the predefined storage-classes (=replication rules) with using AWS API:
+
+
+```
+aws s3 cp --storage-class=REDUCED file1 s3://bucket/file1
+```
+
+
+## [USER] Set the replication for a specific prefix
+
+A user may want to set a custom replication for a specific key prefix. All keys matching that prefix will respect the specified storage class. This operation will not affect keys already in the prefix.
+
+```
+ozone sh prefix setClass --storage-class=REDUCED /vol1/bucket1/tmp
+```
+
+Prefix-level default storage-class can be overridden for ay key, but will be used as default.
+
+## [ADMIN/DEV] Support multiple replication schemes
+
+Today Ozone uses two favor of one replication scheme which are hard coded in the code both on client and server side.
+
+Storage-class abstraction extends this behavior to support any number of replication schemes between server and client and any new replication scheme can be introduced on server side (with code changes!).
+
+Keys (and containers) can be categorized by storage-class which determines the replication scheme. The proposed solution provides the fundamental API which can be used implementing new replication schemes in the future.
+
+Maintaining the storage-class property of the container makes it possible to implement a container allocation in a generic way (we need open containers for each of the storage-classes).
+
+## [ADMIN/USER] Flexible administration
+
+As it's mentioned above, today it's hard to configure the details of the replications for key/bucket level. The only thing what we can define is the replication type for open containers (RATIS/THREE or RATIS/ONE) which determines the later lifecycle of the keys/containers.
+
+Any specific replication configuration can be configured only on cluster level and not on key or container level.
+
+A storage-class can define all the parameters for the specific containers/keys:
+
+Examples for replication mechanism and possible parameters.
+
+
+ | Replication mechanism | Possible parameters     |
+ |-----------------------|------------------------|
+ | Open -> Closed        | ratis quorum (ONE, THREE) and closed replicas
+ | Online, striped EC    | Codec algorithm, number of data and parity instances, size of striping cells |
+ | Offline, container-level EC | Codec algorithm, number of data and parity, conditions to EC a Closed container. 
+
+Definition of a storage-class means a selection of the replication mechanism (first column) and setting the parameters (depends from the selected replication mechanism).
+
+This approach provide an easy-to-use, high-level abstraction for the user, but administrators can configure all the parameters. One replication mechanism (eg. RATIS->CLOSED) might have only limited parameters but others (like EC) have many, but the complexity will be hidden from the users. 
+
+Configuration is not global and not cluster-level, one can have different configuration for different storage-classes (which means for different keys/containers). For example it's possible to define two storage-class with the same mechanism but with different parameters:
+
+| storage class   | mechanism | parameters |
+|-----------------|-----------|------------|
+| NORMAL | RATIS->CLOSED | ratis_quorum = 3, required_closed_replicas = 3 
+| REDUCED | RATIS->CLOSED | ratis_quorum = 1, required_closed_replicas = 1
+
+Users don't need to face with these details as they can use the storage-class (or just use the pre-created buckets and use default storage-class) abstraction.
+
+## [DEV] Give flexibility to the developers
+
+Storage-class abstraction provides an easy way to plug-in newer replication schemes.
+
+Ozone uses containers are the replication units which can contain multiple blocks, but keys with the same replication mechanism should be placed to the same type of containers. Storage-class is a classification for containers which are replicated in the same way.
+
+In case of a new replication mechanism, SCM should handle the new type of the containers (open enough containers from the new type, manage the container type). Storage-class is a one-time improvement which introduces this container grouping and can guarantee the required for each replication mechanism.
+
+Implementing new replication mechanism requires to implement only the specific part (eg. create a new `Manager` in addition to the existing `ReplicationManager` or `PipelineManager`)
+
+## [ADMIN] Better upgrade support
+
+Let's imagine that a new type of Open container replication is introduced (for example *RATIS-STREAM/THREE* instead of *RATIS/THREE`*. If storage-classes are stored with the keys and containers instead of the direct replication rules we can:
+
+ 1. Easily change the replication behavior of existing buckets/keys. (Storage-class could not changed but the replication mechanism or the parameters of the replication can be changed for the groups of keys / containers of that specific storage class.)
+ 2. Turn on experimental features for specific buckets (define new storage-class with experimental replication type for new buckets.)
+
+As it was mentioned earlier, the storage-class of keys couldn't be changed without data movement. But the replication behavior of a storage-class can be changed (depends from allowed migration path).
+
+Example:
+
+ 1. `storage-class=NORMAL` is defined by the administrator as `RATIS/THREE -> CLOSED/THREE` 
+ 2. User creates key `/key1.txt` and `/key2.txt` with `storage-class=NORMAL`
+ 3. Next release introduces `RATIS-STREAM/THREE -> CLOSED` replication mechanism which supports migration from `RATIS/THREE -> CLOSED`
+ 4. As keys are stored in the containers for `storage-class=NORMAL` it's enough to change the definition of `storage-class=NORMAL` to update the replication mechanism of the containers.
+
+And advanced version of the same example:
+
+ 1. After the new version of Ozone (which supports `RATIS-STREAM/THREE -> CLOSED`) the administrator may create a new storage class `EXPERIMENTAL` with the new replication mechanism.
+ 2. New storage class can be used for an experimental bucket or prefix to test if the new replication mechanism is safe and fast enough.
+ 3. If it works well, the definition of `NORMAL` can also be changed as in the previous example.
+
+ Please note that not all the replication behavor changes can be supported. For example an updat of a storage-class definition from `RATIS/THREE -> CLOSED` to `EC` may or may not be supported, depends from the business requirements.
+
+But the storage-class abstraction can help us to implement the upgrade as it classifies the containers based on the required replication mechanism and parameters. 
+
+## [ADMIN] Change the cluster-wide replication
+
+An admin may decide to set a custom policy for an entire cluster.
+
+```
+ozone sh prefix setClass --storage-class=EC_6_3 /
+```
+
+# Unsupported use cases
+
+The following use cases are specifically unsupported.
+
+## [USER] Change the replication policy for a pre-existing key
+
+Changing the replication policy for a pre-existing key will require data movement and re-authoring containers and hence it is unsupported.
+
+## [USER] Defining storage-classes using Hadoop Compatible File System interface
+
+It's not possible to defined storage-class (or any replication rule) with using *Hadoop Compatible File System* interface. However storage-class defined on bucket level (or prefix level) will be inherited, even if the keys are created view the `o3fs://` or `o3s://` interfaces
+
+# The storage-class as an abstraction
+
+After the definition of the use cases, let's talk about abstraction level of the storage-classes in the scope of Ozone.
+
+## Containers in more details
+
+Container is the unit of replication of Ozone. One Container can store multiple blocks (default container size is 5GB) and they are replicated together. Datanodes report only the replication state of the Containers back to the Storage Container Manager (SCM) which makes it possible to scale up to billions of objects.
+
+The identifier of a block (BlockId) contains ContainerId and LocalId (ID inside the container). ContainerId can be used to find the right Datanode which stores the data. LocalId can be used to find the data inside one container.
+
+Today Ozone operates with *Open* and *Closed* containers. For both of them we should define the following behaviors.
+
+ * How to write to the containers?
+ * How to read from the containers?
+ * How to recover / replicate data in case of error
+ * How to store the data on the Datanode (related to the *how to write* question?)
+ 
+**Replication mechanism** is the definition how those Containers are used during the lifecycle of the data. It defines the used states, lifecycle and transitions between the states. 
+
+The high level overview of *Open* containers:
+
+ * **How to write**: Call standard Datanode RPC API on *Leader*. Leader will replicate the data to the followers
+ * **How to read**: Read the data from the Leader (stale read can be possible long-term)
+ * **How to replicate / recover**
+    * Transient failures can be handled by new leader election
+    * Permanent degradation couldn't be handled. (Transition to Closed containers is required)
+
+The high level overview of *Closed* containers:
+
+  * **How to write**: Closed containers are not writeable
+  * **How to read**: Read the data from any nodes (Simple RPC call to the DN)
+  * **How to replicate / recover**
+    * Datanodes provides a GRPC endpoint to publish containers as compressed package
+    * Replication Manager (SCM) can send commands to DN to replicate data FROM other Datanode
+    * Datanode downloads the compressed package and import it
+
+The current replication mechanism is the usage of Open and Closed containers with a one-way transitions between them (in case the container is full or in case of error.) 
+
+ 1. One which `RATIS/THREE` -> `CLOSED/THREE`
+ 2. The other one `CLOSED/ONE` -> `CLOSED/ONE`
+
+## Storage-class
+
+Storage class is a selection of replication mechanism and definition of parameters.
+
+As we saw we have one replication mechanism (which uses Open and Closed containers) with two parameters:
+
+ 1. `ratis_quorum` (can be `ONE` or `THREE`)
+ 2. `required_closed_replicas` (can be `ONE` or `THREE`)
+
+Note: Today the second one is explicitly set by the first one, there is no way to adjust it.
+
+Let's call this replication mechanism (which defines the usage of Open and Closed containers and the transitions) as `NORMAL`
+
+Today's implementation of Ozone can be described with two storage-classes:
+
+![Current replication with Storage class](storage-class-today.png)
+
+Both of these use the same replication mechanism but different parameters. 
+
+| storage class   | mechanism | parameters |
+|-----------------|-----------|------------|
+| NORMAL | NORMAL (RATIS->CLOSED) | ratis_quorum = 3, required_closed_replicas = 3 
+| REDUCED | NORMAL (RATIS->CLOSED) | ratis_quorum = 1, required_closed_replicas = 1
+
+Please note that the storage-class is just an abstraction. After choosing the right replication mechanism and parameters based on storage-class, the replication will be exactly as before
+
+Keys with STANDARD storage-class (*NORMAL* replication with parameters *THREE/THREE*) 
+
+ * *First container type/parameters*: Ratis/THREE replicated containers
+ * *Transitions*: In case of any error or if the container is full, convert to closed containers
+ * *Second container type/parameters*: Closed/THREE container
+
+REDUCED storage-class  (*NORMAL* replication with parameters *ONE/ONE*) 
+
+ * *First container type/parameters*: Ratis/ONE replicated containers
+ * *Transitions*: In case the container is full, convert to closed containers
+ * *Second container type/parameters*: Closed/ONE container
+
+This proposal suggests to introduce an abstraction and name for these two possible scheme. 
+
+In the future Ozone can be improved in two directions:
+
+ 1. Introducing new replication mechanism (like EC)
+ 2. Make existing replication mechanism configurable (for example support  *Ratis/THREE->CLOSED/TWO*) 
+
+Again: the storage-class is a selection of existing replication mechanism with configuration. Both of these challenges can be implemented with the storage-class abstraction.
+
+**Key properties of the storage-class abstraction**:
+
+ * **Storage-class can be defined by configuration**: Storage class is nothing more just the definition of the required replication with parameters.
+ * **Storage-class** is based on existing replication mechanism, implemented in the code. 
+ * **Object creation requires storage class**: Right now we should defined *replication factor* and *replication type* during the key creation. They can be replaced with setting only the Storage class
+ * **Storage-class is property of the containers**: As the unit of replication in Ozone is container, one specific storage-class should be adjusted for each containers.
+ * **Storage-class should be defined for each key** to find the right container to store. But this value can be defined by a bucket/volume/prefix level default.
+ * **Changing definition/configuration of storage class** may be allowed for selected properties. (For example `required_closed_replica` of the normal replication mechanism can be changed: The change will modify the behavior of the Replication Manager, and -- eventually -- the new number of replicas will be provided.)
+ * **Changing storage class of a key** is not possible without moving data between containers.
+ * **Buckets,volumes and prefixes can provide default storage-classes**, to be inherited if not set on the Key level
+
+*Note*: we already support storage-class for S3 objects the only difference is that it would become an Ozone level abstraction and it would defined *all* the container types and transitions.
+
+## Transitions between storage classes
+
+Storage-class is a property of all the *containers*. If a container is tagged with `STANDARD` storage-class, it defines how the container should be replicated (and what are the parameters for that specific replication mechanism).
+
+For example a `STANDARD` container should be replicated with the normal replication mechanism.
+
+The *normal* replication uses:
+
+ 1. `RATIS/THREE` for `Open` containers
+ 2. `CLOSED/THREE` for `Closed` containers
+ 3. `Open` -> `Closed` transition is defined in case of error or if the container is full
+   
+Today this replication mechanism have only one configurable options: use factor *ONE* or *THREE* for both type of containers (Open/Closed). But this can be changed for new (or existing) replication mechanism with introducing more and more parameters.
+
+When keys are created, blocks are assigned from the appropriate containers. **There is no way to change storage-class without moving data** (distcp).
+
+## Storage class on the user interface
+
+Today, we need to define the replication factor (like `THREE`) and replication type (like `RATIS`) during the creation of the key. It's not possible to adjust possible replication rule for the key when it become part of a closed containers.
+
+With storage-class support the users don't need to understand the details of the replication. Users can choose from predefined storage class (Like NORMAL, REDUCED). In the future we can provide support additional, custom storage-classes (using existing replication mechanism with different parameters) Which further improves the flexibility.
+
+## Multi-stage storage-class
+
+There is a very specific use case the "temperature" of data. Key (and containers) can become COLD or HOT over the the time. As we need transitions between the different **state** of the containers, the COLD data should be a new state of a container not a storage-class.
+
+COLD and HOT data are not planned to be added, but a good example for the abstraction level of storage-classes.These are not different replication mechanism, but possible new states in an existing replication workflow.
+
+![Storage class transitions](storage-class-transition.png)
+
+# Implementation changes
+
+The goal with the implementation is to introduce a new abstraction level, but use exactly the same replication mechanism as today.
+
+
+**New interfaces**: To manage the storage-class interfaces we need to define the interfaces:
+
+ * We need configuration classes for the existing replication mechanism to define the replication factor.
+    * For normal `RATIS` replication only the `ratis_quorum`, and the `required_closed_replicas` can be configured and only two tuples are valid `(ONE,ONE)`, `(THREE,THREE)`
+    * `STANDALONE` replication is deprecated but we need to support it until full removal (without parameters)
+ * `StorageClass` is a POJO with the selection of `ReplicationMechanism` and the replication configuration.
+ * We need a `StorageClassRegistry` instance which is available both from client side and server side. Today we don't need to make the configurable from external files: both the existing replication mechanisms can be configured from the code. 
+ * `StorageClassRegistry` will contain the definition of the three initial storage-classes: STANDARD (RATIS/THREE), REDUCED (RATIS/ONE) and LEGACY (STANDALONE)
+ * We need a *legacy mapping table* which maps the `factor/type` from any older requests to `storageClass`! This can help to keep the backward compatibility.
+  
+**Protocol changes**: 
+
+  * `storageClass` field should be added to all the protocols where we use `factor/type` today. Old `factor/type` fields will be kept for compatibility reasons. 
+  * Storage-class will be represented as a custom String in the RPC protocol (in storage-sensitive places it can be stored as an integer with an additional mapping).
+  * There are two protocols which should be changed:
+   * *Key creation* and *Key information* related structures in `OmClientProtocol` should be updated (`KeyInfo`, `MultipartUploadInfo`, `KeyArgs`,... )
+   *  SCMAdmin protocol should be improved to include `storage-class` in `ContainerRequestProto`.
+
+**Ozone Client changes**: Ozone client classes (`OzoneBucket`, `OzoneKey`,...) should be modified to propagate the `storageClass` everywhere instead of `factor/type`. To make it backward compatible we can create legacy mapping table should be used.

Review comment:
       Thanks the question. The implementation details of EC is intentionally removed from this doc as you suggested. This document focuses on the current replication schemes. But EC can be handed in the same way:
   
   The current implementation of OzoneClient calls OM first to create a new key, and uses the returned pipeline information to initialize the right `XCeiverClient` (`XCeiverClientManager.getClient`)
   
   With storage-class the logic is the same, the only required change is adding storage-class to the `CreateKey` request and use it instead of replication/factor. The storage-class will be used in the `AllocateScmBlock` call, and SCM will return with the right pipeline.
   
   But this is an SCM change, client-side it's enough to have the mentioned  change.  
   
   In fact if we implement it once, we don't need to modify the logic for EC, just implement a new `XCeiverClientSpi`. Without storage-class we need to modify the `CreateKey` (or at least adding a new replication type, which doesn't use the factor at all.) 




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