搞了几次花活导致系统引导进不去了，修复好后决定研究下过程，先从 GPT 分区表开始。参考维基百科，GPT 分区表布局如下：
GPT 表以 LBA（Logical Block Address）即逻辑区块地址组织的，查看逻辑区块大小：
❯ lsblk -o NAME,FSTYPE,PHY-SEC,LOG-SEC /dev/nvme0n1 NAME FSTYPE PHY-SEC LOG-SEC nvme0n1 512 512 ├─nvme0n1p1 vfat 512 512 └─nvme0n1p2 ext4 512 512
表示每个逻辑区块为 512 字节，shell 中执行命令：cat /dev/nvme0n1 | head -c 1000000 | hexdump -Cv >> learngpt 直接将硬盘的前一百万个字节导出到文件中，接下来学习 GPT 分区布局。第一个逻辑区块中记录了 GPT 分区表的表头，格式参考维基百科：
分区表表项参考：
分析真实磁盘数据：
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| LBA0 开始 00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000000a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000000b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000000c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000000d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000000e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000000f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000100 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000110 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000120 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000130 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000140 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000150 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000160 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000170 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000180 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000001a0 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 00 00 00 00 00 00 00 00 |................| 000001c0 02 00 ee ff ff ff 01 00 00 00 af 52 77 ee 00 00 |...........Rw...| 000001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 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 45 46 49 20 50 41 52 54 00 00 01 00 5c 00 00 00 |EFI PART....\...| LBA1 起始 00000210 c8 84 00 d9 00 00 00 00 01 00 00 00 00 00 00 00 |................| 00000220 af 52 77 ee 00 00 00 00 00 08 00 00 00 00 00 00 |.Rw.............| 第一个可用于分区的 LBA 是 LBA0x800 00000230 8e 52 77 ee 00 00 00 00 ** ** ** ** ** ** ** ** |.Rw.....********| 打 * 的部分是磁盘 GUID 我藏起来了 00000240 ** ** ** ** ** ** ** ** 02 00 00 00 00 00 00 00 |********........| 00000250 80 00 00 00 80 00 00 00 f8 60 e4 00 00 00 00 00 |.........`......| 00000260 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000270 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000280 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000290 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000002a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000002b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000002c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000002d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000002e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000002f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000300 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000310 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000320 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000330 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000340 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000350 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000360 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000370 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000380 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000390 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000003a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000003b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000003c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000003d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000003e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000003f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000400 28 73 2a c1 1f f8 d2 11 ba 4b 00 a0 c9 3e c9 3b |(s*......K...>.;| LBA2 起始 Entry1 起始 00000410 ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** |****************| 打 * 的部分是分区 GUID 我藏起来了 00000420 00 08 00 00 00 00 00 00 ff 07 08 00 00 00 00 00 |................| 00000430 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000440 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000450 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000460 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000470 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000480 af 3d c6 0f 83 84 72 47 8e 79 3d 69 d8 47 7d e4 |.=....rG.y=i.G}.| Entry2 起始 00000490 ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** |****************| 打 * 的部分是分区 GUID 我藏起来了 000004a0 00 08 08 00 00 00 00 00 ff 4f 77 ee 00 00 00 00 |.........Ow.....| 000004b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000004c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000004d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000004e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 000004f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| ... 00100000 eb 58 90 6d 6b 66 73 2e 66 61 74 00 02 01 20 00 |.X.mkfs.fat... .| 第一个分区开始 00100010 02 00 00 00 00 f8 00 00 20 00 40 00 00 08 00 00 |........ .@.....| 00100020 00 00 08 00 c1 0f 00 00 00 00 00 00 02 00 00 00 |................| 00100030 01 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00100040 80 01 29 73 da 05 0c 65 66 69 2d 62 6f 6f 74 20 |..)s...efi-boot | 00100050 20 20 46 41 54 33 32 20 20 20 0e 1f be 77 7c ac | FAT32 ...w|.| 00100060 22 c0 74 0b 56 b4 0e bb 07 00 cd 10 5e eb f0 32 |".t.V.......^..2| 00100070 e4 cd 16 cd 19 eb fe 54 68 69 73 20 69 73 20 6e |.......This is n| 00100080 6f 74 20 61 20 62 6f 6f 74 61 62 6c 65 20 64 69 |ot a bootable di| 00100090 73 6b 2e 20 20 50 6c 65 61 73 65 20 69 6e 73 65 |sk. Please inse| 001000a0 72 74 20 61 20 62 6f 6f 74 61 62 6c 65 20 66 6c |rt a bootable fl| 001000b0 6f 70 70 79 20 61 6e 64 0d 0a 70 72 65 73 73 20 |oppy and..press | 001000c0 61 6e 79 20 6b 65 79 20 74 6f 20 74 72 79 20 61 |any key to try a| 001000d0 67 61 69 6e 20 2e 2e 2e 20 0d 0a 00 00 00 00 00 |gain ... .......|
需要注意的是 GUID 在小端法中的表示，比如 00000400 行表示 EFI 系统分区的 GUID 的字节序列是 28 73 2a c1 1f f8 d2 11 ba 4b 00 a0 c9 3e c9 3b，但实际上他的值应该是 C12A7328-F81F-11D2-BA4B-00A0C93EC93B，前三部分需要考虑小端法对字节序列的影响，这与 GUID 的字段含义有关，参考：GUID 的基本结构。00000480 行的字节序列是 af 3d c6 0f 83 84 72 47 8e 79 3d 69 d8 47 7d e4，转换为 GUID 即 0FC63DAF-8483-4772-8E79-3D69D8477DE4，正是 Linux 文件系统的标准 GUID。
另外根据 00000220 行的数据可以得出，第一个可用于分区的 LBA 是 LBA0x800（十进制是 2048），查看 fdisk 发现：
❯ fdisk /dev/nvme0n1 Command (m for help): p Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: gpt Device Start End Sectors Size Type /dev/nvme0n1p1 2048 526335 524288 256M EFI System /dev/nvme0n1p2 526336 4000796671 4000270336 1.9T Linux filesystem
第一个分区确实是从第 2048 个区块开始的，为什么第一个分区前要空出来 2048 个分区 * 512 字节每分区 = 1MiB 的空间呢？因为哪怕 GPT 的前 34 个 LBA 全部用上也只占用了 34 LBA * 512 字节每LBA = 17 KiB 的空间，为什么直接空了 1MiB 呢？
以前，硬盘采用 CHS（柱面-磁头-扇区）寻址，
一方面，使用 INT 13h 这样的传统 BIOS API 的老系统和老引导程序要求所有分区必须按柱面对齐。
另一方面，有些系统为了提高性能，避免跨柱面访问硬盘，导致：
MBR 在第一个扇区（属于第一柱面），即 MBR 把第一个柱面占用了；所以第一个分区要从第二个柱面开始。
每个柱面 63 个扇区，即 MBR 占用第一柱面的 63 个扇区，所以第一个分区前会有 63 个扇区的空间。
MBR 大小只有一个扇区（512 字节），第一柱面剩下的空间没有利用，有些引导程序（如 grub）就拿来用了。
随着硬件的发展，许多新硬盘：
1、一个物理扇区有 4096 个字节，即 4KiB
2、使用 LBA 寻址，不再使用 CHS 寻址
如果分区的起始位置未对齐到 4KiB 的整数倍，文件系统的逻辑块可能跨越两个物理扇区。
此时，读写一个逻辑块需要操作两个物理扇区，导致性能下降。
举例：假设分区从第 63 扇区（31.5KiB）开始，其第一个 4K 簇将覆盖扇区 63~70（31.5KiB ~ 35KiB）
导致每个簇读写需操作两个物理扇区，性能损失可达 10%~20%
在保持兼容的基础上为了最大化利用磁盘性能，需要从第 63 扇区（31.5KiB）开始，挑一个能被 4KiB 整除的值就可以了。
可以是 32KiB、64KiB 或者更大的，都行。
为什么最后选择了 1MiB 这个数呢？不知道，得问微软：
自 Windows Vista 起，微软将分区对齐到 1MiB 作为默认设置，以支持高级格式磁盘。这一标准随后被广泛采纳。
主流工具如 fdisk、parted、Disk Utility 也跟进，默认采用 1MiB 对齐，简化用户操作并避免兼容性问题。
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