Reads are complete when the operating system receives the data, while writes are complete when the drive informs the operating system that it has received the data.įor writes, the data may still be in a DRAM cache on the drive or disk controller, depending on your caching policy and hardware. The measurement starts when the operating system sends a request to the drive (or the disk controller) and ends when the drive finishes processing the request. This is often called response time or service time. The first metric is latency, which is simply the time that it takes an I/O to complete. There are actually three main metrics that are most important when it comes to measuring I/O subsystem performance: Latency There are a number of reasons for poor storage performance, but measuring it and understanding what needs to be measured and monitored is always a useful exercise. One of the most common performance bottlenecks that I see as a consultant is inadequate storage subsystem performance.
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