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| <h1>Ozone Manager</h1> |
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| <p>Ozone Manager (OM) is the namespace manager for Ozone.</p> |
| |
| <p>This means that when you want to write some data, you ask Ozone |
| Manager for a block and Ozone Manager gives you a block and remembers that |
| information. When you want to read that file back, you need to find the |
| address of the block and Ozone Manager returns it you.</p> |
| |
| <p>Ozone Manager also allows users to organize keys under a volume and bucket. |
| Volumes and buckets are part of the namespace and managed by Ozone Manager.</p> |
| |
| <p>Each ozone volume is the root of an independent namespace under OM. |
| This is very different from HDFS which provides a single rooted file system.</p> |
| |
| <p>Ozone’s namespace is a collection of volumes or is a forest instead of a |
| single rooted tree as in HDFS. This property makes it easy to deploy multiple |
| OMs for scaling.</p> |
| |
| <h2 id="ozone-manager-metadata">Ozone Manager Metadata</h2> |
| |
| <p>OM maintains a list of volumes, buckets, and keys. |
| For each user, it maintains a list of volumes. |
| For each volume, the list of buckets and for each bucket the list of keys.</p> |
| |
| <p>Ozone Manager will use Apache Ratis(A Raft protocol implementation) to |
| replicate Ozone Manager state. This will ensure High Availability for Ozone.</p> |
| |
| <h2 id="ozone-manager-and-storage-container-manager">Ozone Manager and Storage Container Manager</h2> |
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| <p>The relationship between Ozone Manager and Storage Container Manager is best |
| understood if we trace what happens during a key write and key read.</p> |
| |
| <h3 id="key-write">Key Write</h3> |
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| <ul> |
| <li><p>To write a key to Ozone, a client tells Ozone manager that it would like to |
| write a key into a bucket that lives inside a specific volume. Once Ozone |
| Manager determines that you are allowed to write a key to the specified bucket, |
| OM needs to allocate a block for the client to write data.</p></li> |
| |
| <li><p>To allocate a block, Ozone Manager sends a request to Storage Container |
| Manager (SCM); SCM is the manager of data nodes. SCM picks three data nodes |
| into which client can write data. SCM allocates the block and returns the |
| block ID to Ozone Manager.</p></li> |
| |
| <li><p>Ozone manager records this block information in its metadata and returns the |
| block and a block token (a security permission to write data to the block) |
| to the client.</p></li> |
| |
| <li><p>The client uses the block token to prove that it is allowed to write data to |
| the block and writes data to the data node.</p></li> |
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| <li><p>Once the write is complete on the data node, the client will update the block |
| information on |
| Ozone manager.</p></li> |
| </ul> |
| |
| <h3 id="key-reads">Key Reads</h3> |
| |
| <ul> |
| <li>Key reads are simpler, the client requests the block list from the Ozone |
| Manager</li> |
| <li>Ozone manager will return the block list and block tokens which |
| allows the client to read the data from data nodes.</li> |
| <li>Client connects to the data node and presents the block token and reads |
| the data from the data node.</li> |
| </ul> |
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