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The inodes and the list of blocks that define the metadata of the name system are called the image. NameNode keeps the entire namespace image in RAM. The persistent record of the image stored in the NameNode's local native filesystem is called a checkpoint. The NameNode records changes to HDFS in a write-ahead log called the journal in its local native filesystem. The location of block replicas are not part of the persistent checkpoint.
Each client-initiated transaction is recorded in the journal, and the journal file is flushed and synced before the acknowledgment is sent to the client. The checkpoint file is never changed by the NameNode; a new file is written when a checkpoint is created during restart, when requested by the administrator, or by the CheckpointNode described in the next section. During startup the NameNode initializes the namespace image from the checkpoint, and then replays changes from the journal. A new checkpoint and an empty journal are written back to the storage directories before the NameNode starts serving clients.
For improved durability, redundant copies of the checkpoint and journal are typically stored on multiple independent local volumes and at remote NFS servers. The first choice prevents loss from a single volume failure, and the second choice protects against failure of the entire node. If the NameNode encounters an error writing the journal to one of the storage directories it automatically excludes that directory from the list of storage directories. The NameNode automatically shuts itself down if no storage directory is available.
User applications access the filesystem using the HDFS client, a library that exports the HDFS filesystem interface.Like most conventional filesystems, HDFS supports operations to read, write and delete files, and operations to create and delete directories. The user references files and directories by paths in the namespace. The user application does not need to know that filesystem metadata and storage are on different servers, or that blocks have multiple replicas.
When an application reads a file, the HDFS client first asks the NameNode for the list of DataNodes that host replicas of the blocks of the file. The list is sorted by the network topology distance from the client. The client contacts a DataNode directly and requests the transfer of the desired block. When a client writes, it first asks the NameNode to choose DataNodes to host replicas of the first block of the file. The client organizes a pipeline from node-to-node and sends the data. When the first block is filled, the client requests new DataNodes to be chosen to host replicas of the next block. A new pipeline is organized, and the client sends the further bytes of the file.
The NameNode in HDFS, in addition to its primary role serving client requests, can alternatively execute either of two other roles, either a CheckpointNode or a BackupNode. The role is specified at the node startup. The CheckpointNode periodically combines the existing checkpoint and journal to create a new checkpoint and an empty journal. The CheckpointNode usually runs on a different host from the NameNode since it has the same memory requirements as the NameNode. It downloads the current checkpoint and journal files from the NameNode, merges them locally, and returns the new checkpoint back to the NameNode.
Creating periodic checkpoints is one way to protect the filesystem metadata. The system can start from the most recent checkpoint if all other persistent copies of the namespace image or journal are unavailable. Creating a checkpoint also lets the NameNode truncate the journal when the new checkpoint is uploaded to the NameNode. HDFS clusters run for prolonged periods of time without restarts during which the journal constantly grows. If the journal grows very large, the probability of loss or corruption of the journal file increases. Also, a very large journal extends the time required to restart the NameNode. For a large cluster, it takes an hour to process a week-long journal. Good practice is to create a daily checkpoint.
A recently introduced feature of HDFS is the BackupNode. Like a CheckpointNode, the BackupNode is capable of creating periodic checkpoints, but in addition it maintains an in-memory, up-to-date image of the filesystem namespace that is always synchronized with the state of the NameNode.
The BackupNode accepts the journal stream of namespace transactions from the active NameNode, saves them in journal on its own storage directories, and applies these transactions to its own namespace image in memory. The NameNode treats the BackupNode as a journal store the same way as it treats journal files in its storage directories. If the NameNode fails, the BackupNode's image in memory and the checkpoint on disk is a record of the latest namespace state.
The BackupNode can create a checkpoint without downloading checkpoint and journal files from the active NameNode, since it already has an up-to-date namespace image in its memory. This makes the checkpoint process on the BackupNode more efficient as it only needs to save the namespace into its local storage directories. The BackupNode can be viewed as a read-only NameNode. It contains all filesystem metadata information except for block locations. It can perform all operations of the regular NameNode that do not involve modification of the namespace or knowledge of block locations. Use of a BackupNode provides the option of running the NameNode without persistent storage, delegating responsibility of persisting the namespace state to the BackupNode.
