The two servers are gigabit, and the switch is also gigabit, but why is the transmission only 100Mbps?

Network Configuration Issues: Check the configuration of the network devices to ensure that the network settings for Server A and Server B are correct. This includes checking the Internet Protocol Address, Subnet Mask, Gateway, and other settings. Also, make sure that the network connection between Server A and Server B is correct, for example, check if the cable is loose or damaged.
Network bottlenecks: In some cases, network transfer speeds can be affected by network bottlenecks. For example, if the network connection between Server A and Server B passes through a bottleneck device (such as a router or switch), then the transfer speed may be limited. Please check the configuration and performance of network devices to ensure that they do not become bottlenecks in transfer speeds.
Network driver issues: There may be a problem with the network driver, causing the transfer speed to drop. Please check that the network drivers for Server A and Server B are up to date and try to update the drivers.
Network protocol issues: Please check whether the network protocols used between Server A and Server B are compatible. If not, you may need to change the network protocol or add the necessary protocols.
File system performance issues: File system performance can also affect transfer speed. Please check the file system performance of Server B and try to optimize file system performance.
RAID1 Array Impact: Although you mentioned that Server B has enabled two 2TB RAID1 arrays, RAID1 arrays usually don’t have much of an impact on transfer speed. Make sure the RAID1 array is configured correctly and there are no failures or errors.
Test Method Issues: Finally, make sure that the test method you are using is correct and accurately measures the transfer speed. Sometimes, using incorrect test methods can result in inaccurate test results.