block & sector区别 protocol, command, Memory, sector, number

block :  A number of bytes, basic data transfer unit (SD中最小的读写单位)。
        block length 可以被 SET_BLOCKLEN (CMD16)设置 。
        SDHC and SDXC block length 是固定的512Bytes。
        协议原文：
        7.2.3 Data Read
The SPI mode supports single block read and Multiple Block read operations (CMD17 or CMD18 in the
SD Memory Card protocol). Upon reception of a valid read command the card will respond with a
response token followed by a data token (refer to Figure 7-3). In case of Standard Capacity Card, the
size in the data token is determined by the block length set by SET_BLOCKLEN (CMD16).  In case of
SDHC and SDXC Cards, block length is fixed to  512 Bytes regardless of the block length set by
CMD16.
        sector:  A number of blocks, basic erase unit (SD擦除单位)
        可以设置为N*block
        协议原文：
        SECTOR_SIZE
The size of an erasable sector. The content of this register is a 7-bit binary coded value, defining the
number of write blocks (see WRITE_BL_LEN). The actual size is computed by increasing this number
by one. A value of zero means one write block, 127 means 128 write blocks.
Linux内核文件系统block与硬盘sector关系：        在系统运行过程中，有时会遇到下面打印信息，报告读写某个扇区错误
        kernel: end_request: I/O error, dev sdg, sector 2252148039
kernel: end_request: I/O error, dev sdc, sector 3297222879
        1、这个扇区（sector）的含义是什么？和硬盘上的sector是一回事吗？
        2、Sector和文件系统中的Block有什么关系？
        3、而在我们上层应用读写的是文件内偏移量pos，pos与block/Sector之间有什么关系？
       
        文件偏移量pos，是针对文件本身而言，即文件内的偏移。
        Block是文件系统上的概念，一般文件系统block大小为4K。
        Sector是磁介质硬盘最小单元，一般为512字节。
        Block值一般与sector值是不相等的。