A brief history of the development of the iSCSI storage protocol

iSCSI stands for Internet Small Computer System Interface. It can be seen that iSCSI is SCSI based on Ethernet. SCSI stands for Small Computer System Interface. Many devices use SCSI to exchange data with computers, such as hard disks, floppy drives, optical drives, printers, scanners, etc.

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Next, we will continue to introduce iSCSI related content.

1. SCSI protocol

SCSI was first invented in 1982. There are many different standards along the way, such as SCSI-1, SCSI-2 and SCSI-3. Different standards have different speeds and different hardware interfaces. The following are different SCSI interfaces.

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SCSI is not just an interface, but a complete set of protocols. It implements the protocol for the computer host to communicate with peripheral devices. In this way, the computer host can exchange data with these peripheral devices. Typical examples include exchanging data with hard disks and optical drives.

2. iSCSI protocol

It is not convenient to use such thick cables to transmit data, so iSCSI was born. iSCSI transmits SCSI commands based on Ethernet, which is very convenient to connect to, so it is also convenient to access devices.

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How does Ethernet carry SCSI commands? In fact, the principle is very simple, that is, SCSI commands are encapsulated into data packets and then transmitted through the TCP protocol. As shown in the figure, the so-called iSCSI protocol can be understood as an application layer protocol, and its content is various SCSI commands.

We know that the TCP protocol is based on the CS architecture, and the iSCSI protocol is also based on the CS architecture. The overall architecture of iSCSI is shown in the figure. iSCSI is usually used to access storage devices, with the storage device as the server and the computing server as the client. There are special terms in iSCSI. The client software is usually called the initiator, and the server software is called the target. Based on the iSCSI protocol, when the host is connected to the storage, a disk can be seen on the host, which is no different from a local disk.

In this way, the host can format the file system based on the disk and put data on the disk. When the disk is read or written, the read or write request will be converted into SCSI instructions by the initiator and sent to the server. The server parses and processes it and returns a response to the initiator.

Since the protocol is based on Ethernet, a very complex network can be formed between the host and the storage through the switch, so the host's access to storage resources becomes very flexible.

3. iSCSI based on RDMA

The iSCSI protocol is already very good, but because it is based on the Ethernet protocol, it needs to copy memory between user mode and kernel mode, which not only takes up CPU resources, but also increases IO latency. In order to further improve the performance of iSCSI, iSCSI based on RDMA, iSER, was developed.

iSER stands for iSCSI Extensions for RDMA, which means it is an iSCSI extension protocol based on RDMA. It mainly uses RDMA technology and is essentially an encapsulated SCSI instruction. As shown in the figure, when writing data on the initiator side, the data is directly sent from the memory of the initiator side to the CPU of the target side for processing, and finally persisted to the persistent storage of the target side. In other words, the processing of the CPU on the initiator side is skipped.

Based on the above technology, the delay of the initiator accessing the target device has been greatly reduced, and the performance will naturally be improved accordingly.

4. Performance comparison

We have said a lot of nonsense before, but what is the specific performance like? Let's take a look at the test data of SNIA. The following is the data of IO response delay, and we can see that the delay based on iSER has been greatly reduced.

In terms of bandwidth, iSER has significantly increased it. Therefore, iSCSI based on RDMA technology currently has a very obvious advantage in performance.

This article briefly introduces the development history of iSCSI, starting from SCSI, to iSCSI based on Ethernet, and finally to iSCSI based on RDMA technology. Through the introduction of this article, I hope everyone has a better understanding of iSCSI.