Have you ever wondered how much data written to your Ext4 filesystem since it's first creation time or last mount time. Here is two ext4 sysfs parameters that will tell you the magic numbers you need.
/sys/fs/ext4//lifetime_write_kbytes
/sys/fs/ext4//session_write_kbytes
Here is some more ext4 sysfs paramters and their meanings, but mostly tuning of ext4 multiblock allocator.
ext4 sysfs paramters