Why the return of func Read() with a FileReader is not consistent?

[YYX413]

hello colinmarc, I am encountering a problem with read specific amount of bytes ,say 128MB, from a FileReader.

While the file is 1000MB, I tried to read the file by a loop and 128MB for each iteration except the last one;

but I found that each iteration can only returned 128KB;

and I tried to read 100KB by a loop;

but I found that not each iteration return 100KB consitently, some calls only returned 8192KB;

I found an annotation in block_read_stream.go:

// Always align reads to a chunk boundary. This makes the code much simpler,
// and with readers that pick sane read sizes (like io.Copy), should be
// efficient.

I am wondering if the phenomenon of Read inconsitently is related to the chunk boundaring mentioned above, if it is, I aslo wanna know how that make io.Copy more efficient?

I tried ReadAt also, which can make my need satisfied, but I am still curious about the underlying logic of func Read, thank you!