thanks for all the reply.
I performed some further test on the VM, if i uncheck "snapshot the virtual machine's memory" option, the task could complete very soon, and if doing the snapshot with "snapshot the virtual machine's memory" option check by default the task would take a long time. According to my test, take a snapshot on a powered on VM with 2GB memory takes about 90 seconds, a powered on VM with 4GB memory takes about 4 minutes, a powered on VM with 8GB memory or higher may need more than 7 minutes.
All the VM's swap files were stored in the same position with the VM files in a FC SAN LUN, and the VM is totally idle while taking snapshot.
Does anyone of you has similar test result? Does my vSphere environment going normally?