Hadoop learning (1) HDFS

It lost schoolboy 2020-11-09 13:13:43
hadoop learning hdfs

( One )HDFS

distributed file system , For big data storage . High fault tolerance 、 high reliability 、 High scalability and high throughput .

1. HDFS framework

HDFS yes master/slave Architecture design pattern . One HDFS The cluster has a metadata node (NameNode) And some data nodes (DataNode) form .NameNode It's the master node , It is mainly used to store metadata of management files 【 The metadata size is 150byte namely 8 The metadata is 1K】( File name 、 size 、 Storage path ) And processing requests from clients .DataNode It's the slave node , It is used to manage the data storage of the corresponding node . The file is stored in DataNode On .

2.  Main components

HDFS The data in is stored in the form of data blocks . The default size is 128M.( If a block is less than 128M, It is stored according to the specific size . It doesn't take up the whole space )


Store metadata information and receive client requests .

Metadata : file name 、 file size 、 File storage location, etc . System metadata is stored in fsimage and edits in .

FsImae: System mapping file , It mainly stores source data information .

Edits: Operation log file .HDFS The operation log of the file is saved in it .


Store data in blocks .

Periodically Name Node Send a heartbeat , Reporting on data storage


DN How it works

  1. Data block in DN Store in the form of a file . Contains two files , One of them is the document itself , The other is the verification file of the file ( length , Data blocks 、 The checksum 、 Time stamp )
  2. DN After starting , towards NN register . periodic (1 Hours ) to NN Report block situation .
  3. DN and NN There's a heartbeat mechanism between (3S). If NN Over a period of time (10M) Have not received DN The heart of , The machine is not available .

In order to solve the problem of long system startup time . assist Name Node Implementation work .


Metadata exists in memory , You can read the request quickly . Worry about losing power , So there are backup files on disk .FsImage.


Memory metadata modification , Synchronize updates FsImage Too slow ( Modify the operating ), introduce Edits, Just add , It's very efficient .


SNN effect : System startup process . The system starts ,Name Node Will load FSimage and Edits. After getting the complete metadata information , Will write FSimage in . If the system starts twice for a long time ,edits It's bigger . Merge fsimage and edits It will take time . It takes a long time to start . therefore ,secondNameNode It's to help NamaNode Merge ahead of time FsImage and Edits file .


SNN Workflow :

  1. SNN Intercede NN If you need checkPoint. Returns whether the result is needed or not .
  2. SNN request NN perform Checkpoint
  3. NN Scroll what you're writing Edits journal
  4. adopt Http get Method will mirror the corpus FsImage And compile logs Edits copy to SNN.
  5. SNN Load image and log files into memory and merge . Generate fsImage.chkpoint
  6. adopt Http post take FsImage.chkPoint Send to NN.NN Rename it to FsImage And overlay the original file .


Trigger Second Name Node It's usually time to start or Edits Trigger when the number of bars reaches the threshold .


3.  The reading and writing process of documents

(1)  Read file process

  1. Client to NameNode Send a request to read a file .C->NN
  2. NameNode Return metadata list ( The document is divided into several parts Block, Every Block Corresponding to a list of files )
  3. Through each Block, according to Block List of metadata for , Select the nearest node sent DataNode request . Establishing a connection .
  4. After obtaining the data , Shut down and DataNode The connection of . Deal with the next Block Node download .
  5. Close the mission .

(2)  Write file process

  1. Client to NamaNode Send write file request .C->NN
  2. NameNode Verify permissions , Whether the document exists, etc . Returns whether you can upload .
  3. The client loops through each block Block, towards NN Send a request . Get the current Block List of metadata for .[ds1ds2ds3]
  4. For the current Block, Select recent DataNode ds1, Use FSDataOutPutStream Establishing a connection , Write data .
  5. The backup mechanism is responsible for backing up the written data to other nodes ds1 Data writing ds2,ds2 Data writing ds3.
  6. To complete all Block After writing , Close the mission .
4.  Knowledge point

(1)  Network topology : Node calculation

Distance between nodes : The two nodes arrive at the sum of the nearest common ancestor .

(2) HADOOP Replica node selection

The first copy , If the client is on the node , It's the current node , If the client is not on the node , The first copy is optional .

The second copy is in the same rack as the first , On different nodes .

The third copy is different from the first two frames , Nodes are random .

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