Disk activity is much slower than reading data from RAM. With today’s performance characteristics, reading from DRAM takes around 100 nanoseconds whereas reading from the physical drive is between 10 microseconds (for SSD) up to 10 milliseconds (for HHD). This is up to 100,000 times slower than accessing the random access memory. Reading from the L1 cache is even faster and can take 3 CPU cycles which is less than 1 nanosecond. Therefore, every read from a physical drive is a tremendous performance hit and should be avoided. In this blog post, we are going to see how to debug scenarios where we can’t utilize cached data and need to read from the physical drive. We’re going to see why it’s important, what to look for, and what tools and extensions to use.

How Databases Read Data

Databases are well aware of performance issues when reading data directly from the hard drive. Therefore, they incorporate many sophisticated techniques to boost performance and cache data where possible. Let’s see how the database can access the data and what happens next.