elek commented on a change in pull request #1419:
URL: https://github.com/apache/ozone/pull/1419#discussion_r520633410
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File path: hadoop-hdds/docs/content/design/storage-class.md
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@@ -19,10 +19,331 @@ 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 data
copy and read only.
+
+In this document a new level of abstraction is proposed: the *storage class*
which defines which type of containers should be used and what type of
transitions are supported.
+
+# 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 spefific terms when key is created:
+
+```
+ozone sh key put --replication=THREE --type=RATIS /vol1/bucket1/key1
source-file.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 keys are only for the *open* containers. There is no easy way
to add configuration which can be used later during the lifecycle of
containers/keys. (For example to support `Ratis/THREE` --> `Ratis/TWO`)
+
+With the storage-class abstraction the complexity of configuration can be
moved to the admin side (with more flexibility). And user should choose only
from the available storage-classes (or use the default one).
+
+Instead of the earlier CLI 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 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 (question: consider
supporting this with data movement?)
+
+```
+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 there are two replication schemes which are hard coded in the code.
Storage-class abstraction extends this behavior to support any number of
replication schemes.
+
+Keys (and containers) can be categorized by storage-class which determines the
replication scheme.
+
+## [ADMIN/USER] Flexible administrations
+
+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 level.
+
+A storage-class can define all the parameters for the spefific containers/keys:
+
+As an example this could be a storage-class definitions:
+
+```
+name: STANDARD
+states:
+ - name: open
+ replicationType: RATIS
+ repliationFactor: THREE
+ - name: closed
+ replicationType: COPY
+ repliationFactor: TWO
+ rackPolicy: different
+ transitions:
+ - target: ec
+ trigger:
+ ratio: 90%
+ used: 30d
+ - name: ec
+ codec: Reed-Solomon
+ scheme:
+ data: 6
+ parity: 3
+```
+
+This defines a replication scheme where only two replicas are enough from
closed containers, and container will be erasure encoded under the hood if the
90% of the content is not used in the last 30 days.
+
+Please note that:
+
+ * All the low-level details of the replication rules can be configured here
by the administrators
+ * Configuration is not global and not cluster-level, one can have different
configuration for different storage-classes (which means for different
keys/containers)
+ * Users dont' 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. New type of replications (like EC) can be supported easily as the
system will be prepared to allocate different type of containers.
+
+## [ADMIN] Better upgrade support
+
+Let's imagine that a new type of Open container replication is introduced
(`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 replicaiton method of existing buckets/keys
+ 2. Turn on experimental features for specific buckets
+
+
+## [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 reauthoring 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
+
+The previos section explained some user facing property of the storage-class
concept. This section explains the concept compared to the existing Ozone
design.
+
+## 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) containers 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.
+
+Container type defines the following:
+
+ * 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?)
+
+THe current definition of *Ratis/THREE* is the following (simplified version):
+
+ * **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 current definitions of the *Closed/THREE*:
+
+ * **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 definitions of *Closed/ONE*:
+ * **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**: No recovery, sorry.
+
+
+## Storage-class
+
+Let's define the *storage-class* as set of used container **types and
transitions** between them during the life cycle of the containers.
+
+The type of the Container can be defined with the implementation type (eg.
Ratis, EC, Closed) and with additional parameters related to type (eg.
replication type of Ratis, or EC algorithm for EC containers).
+
+Today's implementation of Ozone can be described with two storage-classes:
+
+
+
+The definition of STANDARD Storage class:
+
+ * *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
+
+The definition of REDUCED Storage class:
+
+ * *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 the two possible
scheme. Later we can define other storage classes as well. For example we can
define (Ratis/THREE --> Closed/FIVE) storage class, or more specific containers
can be used for Erasure Coding or Random Read/Write.
+
+With this approach the storage-class can be an adjustable abstraction to
define the rules of replications. Some key properties of this approach:
+
+**Key properties of the storage-class abstraction**:
+
+ * **Storage-class can be defined by configuration**: Storage class is nothing
more just the definition of rules to store / replicate containers. They can be
configured in config files and changed any time.
+ * **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 defined on key** (and container) **level**
+ * **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.
+ * **Changing definition/configuration of storage class** is allowed for most
of the properties. The change will modify the behavior of the Replication
Manager, and -- eventually -- the containers will be replaced in a different
way.
+ * **Changing storage class of a key** is not possible wihout moving data
between containers
+ * **Buckets and volumes 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.
+
+# Storage-class inside Ozone
+
+First of all, we can configure different replication levels easily with this
approach (eg. Ratis/THREE --> Closed/TWO). Ratis need quorum but we can have
different replication number after closing containers.
+
+We can also define topology related transitions (eg. after closing, one
replica should be copied to different rack) or storage specific constrains
(Right now we have only one implementation of the storage: `KeyValueContainer`
but we can implement more and storage class provides an easy abstraction to
configure the required storage).
+
+Datanode also can provide different disk type for containers in a certain
storage class (eg. SSD for fast access).
+
+## 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 transitions should be done (in case of predefined
conditions).
+
+For example a `STANDARD` container should be replicated with `RATIS/THREE`,
and closed in case the container is full. In closed *state*, the container
should be replicated with the standard closed container replication.
+
+When keys are created, blocks are assigned from the appropriate containers.
**There is no way to change storage-class without moving data* (distcp).
+
+bucket.storage-class := STANDARD --> REDUCED
+
+container=REDUCED
+
+key1 (storage-class=REDUCED)
+key2 (straoge-class=REDUCED)
+
+REDUCED: (RATIS/ONE -- CLOSED/ONE) --> (RATIS/TWO -- CLOSED/TWO)
+
+## 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.
+
+Therefore the STANDARD storage class can be modified to support three states:
OPEN, WARM(=CLOSED), COLD. Transition between WARM and CLOSED can be managed by
SCM (if container is not used, SCM can request to enable erasure coding). Or
manually (one specific container can be forced to Erasure Coding / COLD state.
Only useful for administrators)
+
+
+
+## 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 udnerstand the details of
the replication. Users can choose from the well-known or custom-defined
storage-classes.
+
+# Implementation changes
+
+**Phase 1**: while storage class abstraction can provide a very generic
framework for replication, in the first implementation It's proposed to keep it
as simple as possible:
+
+ * Don't introduce any new custom container state
+ * Don't introduce any new transitions
+ * Don't introduce any new custom storage classes
+
+But we can start to use the abstraction for the existing implementation:
+
+ * We will implement two, hard-coded storage-classes (STANDARD, REDUCED) which
can configure the behavior of existing replication schemes
+ * Ozone Client will be changed to support `--storage-class` instead of
`--type` and `--factor`
+ * `ofs://` and `o3fs://` doesn't require any modification
+ * Storage class will be used as a custom String in RPC protocol (in
storage-sensitive places it can be stored as an integer with an additional
mapping)
+ * All the replication logic (PipelineManager/ReplicationManager) will work
exactly as before. Storage-class will be resolved to the required replication
config. Pipelines will have the same type as before (eg. Ratis/THREE)
+ * SCM allocateContainer logic should be changed. Right now it creates a
predefined number of Open containers for each `owner` (high level application).
It can be extended easily to create open containers for each storage-classes
+ * Having slightly more open containers doesn't make any difference (5-10 more
open containers per disk vs. the 5000-10000 existing containers)
+
+# Backward compatibility
+
+ * In allocateBlock and intra-service communication we can replace replication
type and replication factor with storage-class
+ * In client API replication factor and replication type will be optional
+ * Legacy type/factor definitions will be mapped to real storage classes under
the hood (which defines the real replication scheme). For example `Ratis/THREE
--> STANDARD`
+
Review comment:
I wrote it down in more details. (Exactly as today but replication
factor is coming from the storage-class configuration for each containers)
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