读写速度不同时HDD平均存取+搜索时间计算公式问询

Great question—this is a common edge case when refining benchmarking formulas, so let's walk through how to adapt your original equation for unequal read/write speeds.

First, let's recap the logic behind your original formula #

When read and write speeds are identical, your formula works because total copy time breaks down into:

• Time to read the entire file: file_size / read_speed
• Time to write the entire file: file_size / write_speed (same as read time here)
• Overhead from HDD access/seek operations: every time the HDD fills or empties its internal buffer, it needs to perform an access+seek cycle. For a file size larger than the buffer, this happens 2 * (file_size / HDD_buffer_size) times (once per buffer block for reading, once for writing).

The adapted formula for asymmetric read/write speeds #

When read and write speeds differ, we just need to split the read/write time components instead of doubling one value. Here's the corrected equation:

average_access_seek_time = (total_copy_time - (file_size / read_speed + file_size / write_speed)) / (2 * (file_size / HDD_buffer_size))

Why this works #

Let's break down each part:

1. total_copy_time: The real-world time measured during your benchmark
2. file_size / read_speed: Pure time spent reading data from the HDD to memory (no seek/access overhead)
3. file_size / write_speed: Pure time spent writing data from memory to the HDD (no seek/access overhead)
4. 2 * (file_size / HDD_buffer_size): Total number of access+seek cycles needed—each buffer-sized chunk of data requires one seek/access to read, and one to write.

When read_speed = write_speed, this simplifies exactly to your original formula, which confirms it's a valid generalization.

Quick best practices to improve accuracy #

• Use a test file significantly larger than the HDD's internal buffer (e.g., 10x+ the buffer size) to minimize the impact of edge cases with partial buffer fills
• Run multiple tests and take the average to account for background IO fluctuations
• Ensure the test partition is fully defragmented (as you already planned) to avoid extra seek overhead from scattered data

内容的提问来源于stack exchange，提问作者Daro