Analysis of common storage types and FTP active and passive modes

One Common storage types

There are three types of storage ：

• Direct Attached Storage （Direct-Attached Storage）DAS

DAS Storage is the most common way of storage , Especially in the application of small and medium-sized enterprises .PC Hard disk or only one external SCSI Interface JBOD All belong to DAS framework .DAS It means that the storage device is directly connected to the server bus , The storage device is only connected to a separate host , Other hosts cannot use this storage device .DAS The connection channel between the storage device and the server host usually adopts SCSI Connect ,DAS Storage devices are mainly disk arrays （RAID: Redundant Arrays of Independent Disks）、 Disk cluster （JBOD:Just a Bunch Of Disks） etc. .

• Network Attached Storage （Network-Attached Storage）NAS

NAS Storage is the storage device through the standard network topology ( Like Ethernet ) Add to a group of computers . And DAS as well as SAN No 
 Same as ,NAS It's a file level storage method . use NAS More functions are used for file sharing .
NAS Storage is also often referred to as additional storage , seeing the name of a thing one thinks of its function , It's the storage device through the standard network topology ( Ethernet, for example ) add to 
 To a group of computers .NAS It's a file level storage method , Its focus is to help working groups and departmental organizations solve the problem of rapidly increasing storage 
 Demand for capacity . Now more parents use NAS More functions are used for document sharing 、 Picture sharing 、 Movie sharing and so on , And with it 
 With the development of Cloud Computing , some NAS Manufacturers have also launched cloud storage capabilities , It greatly facilitates the use of enterprises, relatives and other individual users .
NAS Products are truly plug and play products .NAS Devices generally support multiple computer platforms , Users can enter the same network through the network support protocol 
 Documents , thus NAS The equipment can be used for mixing without modification Unix/Windows NT In LAN , meanwhile NAS It's very flexible .
 but NAS There is a key issue , That is, the bandwidth consumption in the backup process . And will backup the data stream from LAN Storage area network transferred out of 
（SAN） Different ,NAS Still using the network for backup and recovery .NAS  One drawback of is that it will store transactions from parallel SCSI Connection transfer 
 On the Internet . That is to say LAN In addition to having to handle normal end-user transport streams , You must also deal with storage operations that include backup operations 
 Disk request .

• Storage area network （Storage Area Network）SAN

 Storage area network , This is to connect the storage array and server host through fibre channel or Ethernet switch , Finally become a dedicated 
 Storage networks .SAN After more than ten years of historical development , It's quite mature , Become the industry's de facto standard （ But optical fiber switching of various manufacturers 
 The technology is not exactly the same , Its server and SAN Storage has compatibility requirements ）.
SAN Provides an alternative to existing LAN Easy way to connect , And through the same physical channel support widely used SCSI and IP agreement .
SAN Not subject to the current mainstream 、 be based on SCSI The layout constraints of the storage structure . Of particular importance , With the explosive growth of storage capacity ,
SAN Allow enterprises to increase their storage capacity independently .SAN The architecture allows any server to connect to any storage array , So no matter 
 Put the data there , The server can directly access the required data . Because of the optical fiber interface ,SAN It also has higher bandwidth .
 Today, SAN Solutions usually take two forms ： Fibre channel and iSCSI Or based on IP Of SAN, That is to say FC SAN
 and IP SAN. Fibre channel is SAN The most familiar type of solution , however , recently , be based on iSCSI Of SAN
 Solutions are starting to appear on the market in large numbers , Compared to fibre channel technology , This technology has good performance , And the price is low 
 Lian .
SAN The advantages of ：
 As storage capacity grows ,SAN Allow enterprises to increase their storage capacity independently .
SAN Allow any server to connect to any storage array （ The advantage is ： No matter where the data is , Servers can directly access the required 
 data ）
 Due to the use of optical fiber interface ,SAN With higher bandwidth . except FC Connect ,SAN Connection and ISCSI（SCSI over IP） as well as 
SAS（Serial Attached SCSI） Interface .
 Optical fiber interface can provide 10 The length of the connection that is so long and so far , Very easy to physically separate storage 

Two FTP Active passive mode

File transfer protocol ：File Transfer Protocol One of the three early application level protocols , be based on C/S structure

Two channel protocol ： control （ command ） Connection and data connection

 Control connection ： For transmission FTP command , Such as ： Delete file 、 Rename file 、 Download the file 、 List the contents 、 Get file information, etc 
 Data connection ： The real data transmission is completed through data connection 

By default FTP Active mode , The server 21 Port as command port ,20 Port is data port . But there's a difference in passive mode .

Active mode ：

 First step , The client uses the port N Connect FTP Command port of the server 21, Establish a control connection and tell the server that I have the data port open N+1.
 The second step , After the control connection is established , The server will use data ports 20, Active client connection N+1 Port to establish a data connection . This is it. FTP Active mode connection process .
 Here we need to add , In fact, the command port and data port of the client are not written in some articles N and N+1 The relationship between , The two ports are close ..

Passive mode ：

 First step , The command port of the client N Active connection to server command port 21, And send the PASV command , Tell the server to use “ Passive mode ”, After the successful establishment of the control connection , Server opens a data port P, adopt PORT The order will P The port tells the client .
 The second step , The data port of the client N+1 To connect to the server's data port P, Establish data connection .
 There are two points to add 
   First of all , In fact, the command port and data port of the client are not written in some articles N and N+1 The relationship between , The two ports are close ;
   second , Data port of the server P Is random , This client is connected to 6008 port , Another connection that might be used is 7009, however P The range of ports can be set .

Active and passive modes , In the data connection , If the server actively connects , Active mode ; Server passive connection , Passive mode .