分类： LINUX
2017-06-10 11:41:17
用parted工具给硬盘设备文件，创建一个msdos标签。
[root@(none) /home]$ ./parted --version
parted (GNU parted) 3.1
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Written by <
[root@(none) /home]$ ./parted /dev/sda
GNU Parted 3.1
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) mklabel msdos
mklabel msdos
(parted) quit
quit
Information: You may need to update /etc/fstab.
[root@(none) /home]$ hexdump -C -n 512 /dev/sda
00000000 fa b8 00 10 8e d0 bc 00 b0 b8 00 00 8e d8 8e c0 |................|
00000010 fb be 00 7c bf 00 06 b9 00 02 f3 a4 ea 21 06 00 |...|.........!..|
00000020 00 be be 07 38 04 75 0b 83 c6 10 81 fe fe 07 75 |....8.u........u|
00000030 f3 eb 16 b4 02 b0 01 bb 00 7c b2 80 8a 74 01 8b |.........|...t..|
00000040 4c 02 cd 13 ea 00 7c 00 00 eb fe 00 00 00 00 00 |L.....|.........|
00000050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000001b0 00 00 00 00 00 00 00 00 5a 58 03 00 00 00 00 00 |........ZX......|
000001c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa |..............U.|
00000200
可见msdos标签，写在硬盘第一扇区，55 aa是其的标记，(55 aa == 01010101 10101010)使用比的是较有序的01串。
这个扇区当然不只是放个标签用的，还充当引导扇区的作用，就是所谓的MBR(Master Boot Record)。其数据结构如下：
在linux代码中的头文件有定义
./linux-2.6.18/fs/partitions/efi.h中有
typedef struct _legacy_mbr {
u8 boot_code[440];
__le32 unique_mbr_signature;
__le16 unknown;
struct partition partition_record[4]; // 分区表也叫DPT(Disk Partition Table)
__le16 signature;
} __attribute__ ((packed)) legacy_mbr;
其中的struct partition在
./linux-2.6.18/include/linux/genhd.h
struct partition {
unsigned char boot_ind; /* 0x80 - active */
unsigned char head; /* starting head */
unsigned char sector; /* starting sector */
unsigned char cyl; /* starting cylinder */
unsigned char sys_ind; /* What partition type */
unsigned char end_head; /* end head */
unsigned char end_sector; /* end sector */
unsigned char end_cyl; /* end cylinder */
__le32 start_sect; /* starting sector counting from 0 */
__le32 nr_sects; /* nr of sectors in partition */
} __attribute__((packed));
sizeof (legacy_mbr)=512刚好填满一扇区。
从分区表数据结构中可以看出，同时包含C/H/S和LBA的寻址模式，start_sect就是LBA的开始地址。注意sys_ind这个值是分区类型，而不一定是分区上的文件系统类型，如果没有代码的话可以用fdisk列出各自代表什么意思。下面增加两个分区
[root@(none) /home]$ ./parted /dev/sda
GNU Parted 3.1
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) mkpart primary fat32 1MB 4GB
(parted) mkpart extended 6GB 10GB
(parted) quit
quit
Information: You may need to update /etc/fstab.
然后再看看第一扇区的数据。
[root@(none) /home]$ hexdump -C -n 512 /dev/sda
00000000 fa b8 00 10 8e d0 bc 00 b0 b8 00 00 8e d8 8e c0 |................|
00000010 fb be 00 7c bf 00 06 b9 00 02 f3 a4 ea 21 06 00 |...|.........!..|
00000020 00 be be 07 38 04 75 0b 83 c6 10 81 fe fe 07 75 |....8.u........u|
00000030 f3 eb 16 b4 02 b0 01 bb 00 7c b2 80 8a 74 01 8b |.........|...t..|
00000040 4c 02 cd 13 ea 00 7c 00 00 eb fe 00 00 00 00 00 |L.....|.........|
00000050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000001b0 00 00 00 00 00 00 00 00 b1 ed 06 00 00 00 00 20 |............... |
000001c0 21 00 0c 57 71 e6 00 08 00 00 00 30 77 00 00 73 |!..Wq......0w..s|
000001d0 9b d9 0f fe ff ff 00 d0 b2 00 00 38 77 00 00 00 |...........8w...|
000001e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa |..............U.|
00000200
多 了个分区表，有两条分区记录。mbr.partition_record[0].sys_ind==0x0c表示W95 FAT32& nbsp;(LBA)，所以将mbr.partition_record[0].CHS相关的那些位清零也能正常工作，将 mbr.partition_record[0].LBA相关的那些位清零则不能。若 mbr.partition_record[0].sys_ind==0x0b应该是CHS模式的，后来的ext2/ntfs等文件系统只有LBA模式 的。 C/H/S的值并非上面的mbr.partition_record[0].head/sector/cyl，因为Cylinder的值借用了 Sector值的高2bit作为它自己的高2bit，详细的算法见parted-3.1/libparted/labels/dos.c里面实现的 chs_get_cylinder, chs_get_head, chs_get_sector, chs_to_sector, sector_to_chs 这几个函数。
mbr.partition_record[1].sys_ind==0x0f表示W95 Ext'd (LBA)是个扩展分区。
mbr分区表决定硬盘顶多能有四个主分区(也就只能装4个操作系统)，有了扩展分区后可以有3个主分区+1个扩展分区( 上面能做N个逻辑分区)，突破了分区数量的限制。然而LBA的2T硬盘容量限制，还是需要GPT(GUID Partition Table)分区模式。如下图：LBA0-N对应的就是扇区单位。
其数据结构也是在./linux-2.6.18/fs/partitions/efi.h中定义
typedef struct _gpt_header {
__le64 signature;
__le32 revision;
__le32 header_size;
__le32 header_crc32;
__le32 reserved1;
__le64 my_lba;
__le64 alternate_lba;
__le64 first_usable_lba;
__le64 last_usable_lba;
efi_guid_t disk_guid;
__le64 partition_entry_lba;
__le32 num_partition_entries;
__le32 sizeof_partition_entry;
__le32 partition_entry_array_crc32;
u8 reserved2[GPT_BLOCK_SIZE - 92];
} __attribute__ ((packed)) gpt_header;
typedef struct _gpt_entry_attributes {
u64 required_to_function:1;
u64 reserved:47;
u64 type_guid_specific:16;
} __attribute__ ((packed)) gpt_entry_attributes;
typedef struct _gpt_entry {
efi_guid_t partition_type_guid;
efi_guid_t unique_partition_guid;
__le64 starting_lba;
__le64 ending_lba;
gpt_entry_attributes attributes;
efi_char16_t partition_name[72 / sizeof (efi_char16_t)];
} __attribute__ ((packed)) gpt_entry;
有些变量类型是在./linux-2.6.18/include/linux/efi.h中定义。
很明显，GPT模式的LBA采用的是64位寻址，所以理论上支持(2^64 * 512)的容量，克服了2TB显示。
最大支持128个分区，4条分区记录就占用了一个扇区确实开销很大。需要特别留意的是
1、gpt_header.disk_guid, gpt_entry.unique_partition_guid都是独一无二的，因为从GPT全称看就知道，是靠这个来识别的。
2、gpt_header.num_partition_entries这个值一般等于128，分区记录的总条数，并非硬盘上真正有效的分区个数。
3、gpt_header.header_crc32这个值在计算前需先将自己置零。
4、Secondary GPT Header并非Primary GPT Header的简单备份，它描述的自己的Entry在哪等信息，尽管Entry内容是相同的。
详细的实现代码参考parted-3.1/libparted/labels/gpt.c
经有限的测试，发现没有Secondary GPT Header也能正常使用的。
给硬盘增加个gpt标签
[root@(none) /home]$ ./parted /dev/sda
GNU Parted 3.1
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) mklabel gpt
mklabel gpt
[root@(none) /home]$ hexdump -C -n 1024 /dev/sda
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000001c0 01 00 ee fe ff ff 01 00 00 00 2f 60 38 3a00 00 |........../`8:..|
000001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa |..............U.|
00000200 45 46 49 20 50 41 52 54 00 00 01 00 5c 00 00 00 |EFI PART....\...|
00000210 3c 2d ba 4000 00 00 00 01 00 00 00 00 00 00 00 |<-.@............|
00000220 2f 60 38 3a 00 00 00 00 22 00 00 00 00 00 00 00 |/`8:....".......|
00000230 0e 60 38 3a 00 00 00 00 5c 66 43 b6 17 c6 5f 41 |.`8:....\fC..._A|
00000240 84 1e d0 de 15 8e fd f2 02 00 00 00 00 00 00 00 |................|
00000250 80 00 00 0080 00 00 00 86 d2 54 ab00 00 00 00 |..........T.....|
00000260 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000400
可以看出第一扇区(LBA0)依然是MBR，后面再分析。第二个扇区(LBA1)开始是45 46 49 20 50 41 52 54这
串是GPT标记，因为其最早是EFI(Extensible Firmware Interface)项目中使用，所以这么称呼，上
面的数据结构在efi.h中也是这个原因的。虽然没有分区，但gpt_header.num_partition_entries =&
nbsp;80 00 00 00，Secondary GPT Header先不管它。下面给它增加一个分区
(parted) mkpart primary ext2 10MB 20GB
mkpart primary ext2 10MB 20GB
[root@(none) /home]$ hexdump -C -n 1536 /dev/sda
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000001c0 01 00 ee fe ff ff 01 00 00 00 2f 60 38 3a 00 00 |........../`8:..|
000001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa |..............U.|
00000200 45 46 49 20 50 41 52 54 00 00 01 00 5c 00 00 00 |EFI PART....\...|
00000210 96 80 72 9700 00 00 00 01 00 00 00 00 00 00 00 |..r.............|
00000220 2f 60 38 3a 00 00 00 00 22 00 00 00 00 00 00 00 |/`8:....".......|
00000230 0e 60 38 3a 00 00 00 00 5c 66 43 b6 17 c6 5f 41 |.`8:....\fC..._A|
00000240 84 1e d0 de 15 8e fd f2 02 00 00 00 00 00 00 00 |................|
00000250 80 00 00 00 80 00 00 00 3e d4 69 7500 00 00 00 |........>.iu....|
00000260 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000400 a2 a0 d0 eb e5 b9 33 44 87 c0 68 b6 b7 26 99 c7 |......3D..h..&..|
00000410 92 e7 b6 5c 70 67 ad 4d 9f d2 da f9 64 7b 56 bd |...\pg.M....d{V.|
00000420 00 50 00 00 00 00 00 00 ff 07 54 02 00 00 00 00 |.P........T.....|
00000430 00 00 00 00 00 00 00 00 70 00 72 00 69 00 6d 00 |........p.r.i.m.|
00000440 61 00 72 00 79 00 00 00 00 00 00 00 00 00 00 00 |a.r.y...........|
00000450 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000600
分 区的信息从第三扇区(LBA2，确切的讲是gpt_header.partition_entry_lba)开始，同时分区的修改带动 gpt_header.partition_entry_array_crc32的变化，又引起了gpt_header.header_crc32的变 化，真不简单啊！
