An e-commerce company configures the highest I/O priority for its main online transaction services, and configures medium and low priorities for its instant messaging and archiving services respectively. The I/Os ①②③④⑤⑥ need to be written at the same time. Of these, ③ and ⑤ are online transaction services, ② and ⑥ are instant messaging services, and ① and ④ are archiving services. Which I/O write sequences are correct?

• A. ③⑤①④⑥②
• B. ③⑤②⑥①④
• C. ①②③④⑤⑥
• D. ③④⑤⑥①②

Answer: (B)

I/O scheduling uses priority to decide which writes go first. The online transaction services have the highest priority, instant messaging is medium, and archiving is low. To honor these priorities, all high-priority writes should be issued before any medium-priority writes, and those before any low-priority ones. Since there are two high-priority writes, they come first; then the two medium-priority writes; finally the two low-priority writes. So the correct sequence places the online transaction writes first, followed by the instant messaging writes, and then the archiving writes. This ordering aligns with the priority configuration and minimizes latency for critical online transactions.