what is db_block_gets,db_block_gets,db_block_gets

 i have a question ,

i did a test in  11G in a session :

insert into t select form t; 

 -about 1548800*2  rows inserted, cousume about 102S;

i monior v$sess_io,

find consumer data is about

 block_gets consistent_gets physical_reads block_changes consistent_changes

 358658          122810              27487                300733            1000

i don't kown what is db_block_gets,db_block_gets,db_block_gets unit, is that use db_block_size as unit

so i can guess cosumue data  is  abourt

358658  * 8192 byte         122810*8192 BYTE         27487*8192BYTE        300733*8192BYTE   1000*8192byte

but i use os command such as sar/IOSTAT  ,monitorr  disk read/write is abort  15M * 102s=1G,

so why the two is different ?  can give us a explain!