Is MySQL sure about the sequential read is really sequential in SSD

[haochen he]

In SSD, overwriting a data file means SSD has to first erase it and then write data. This can change the data layout for the data file.

MySQL has some policies to optimize random IO, e.g., read-ahead (pre-fetching). Read-ahead policy will pre-read the remaining x pages in an extent if prior y pages are read sequentially. From the perspective of MySQL, using consecutive offsets for `pread` can make reads sequential. But I doubt is it true for a SSD.

This point to another important question: does an extent (usually contains 64 pages) in MySQL must represent consecutive space for SSD.