关于获取磁盘映像并缩小根（仿生）文件系统分区然后截断映像以删除成为可用空间的部分的大量信息。喜欢 https://softwarebakery.com//shrinking-images-on-linux
所以基本上分为三个步骤。使用 resize2fs 缩小分区上的文件系统，然后也减小分区的大小。然后最后删除图像文件中现在未使用的空间。
这是我遇到问题的第三部分。每次我尝试 dd 或 truncate 时，图像的所有分区都会被删除，就好像图像文件中的 GPT 分区表被丢弃一样。
这是原始图像
GPT fdisk (gdisk) version 1.0.3
Partition table scan:
MBR: protective
BSD: not present
APM: not present
GPT: present
Found valid GPT with protective MBR; using GPT.
Disk rock64-base.img: 30310400 sectors, 14.5 GiB
Sector size (logical): 512 bytes
Disk identifier (GUID): 159DCEDE-DBEA-4657-96D9-2CE178A96B7E
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 30310366
Partitions will be aligned on 64-sector boundaries
Total free space is 30 sectors (15.0 KiB)
Number Start (sector) End (sector) Size Code Name
1 64 8063 3.9 MiB 8300 loader1
2 8064 8191 64.0 KiB 8300 reserved1
3 8192 16383 4.0 MiB 8300 reserved2
4 16384 24575 4.0 MiB 8300 loader2
5 24576 32767 4.0 MiB 8300 atf
6 32768 262143 112.0 MiB 0700 boot
7 262144 30310366 14.3 GiB 8300 root
然后在前两个步骤之后
Found valid GPT with protective MBR; using GPT.
Disk test.img: 30310400 sectors, 14.5 GiB
Sector size (logical): 512 bytes
Disk identifier (GUID): 159DCEDE-DBEA-4657-96D9-2CE178A96B7E
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 30310366
Partitions will be aligned on 64-sector boundaries
Total free space is 23555002 sectors (11.2 GiB)
Number Start (sector) End (sector) Size Code Name
1 64 8063 3.9 MiB 8300 loader1
2 8064 8191 64.0 KiB 8300 reserved1
3 8192 16383 4.0 MiB 8300 reserved2
4 16384 24575 4.0 MiB 8300 loader2
5 24576 32767 4.0 MiB 8300 atf
6 32768 262143 112.0 MiB 0700 boot
7 262144 6755394 3.1 GiB 8300 primary
可以看到根文件系统分区已经缩减到3.1G
我可以很好地加载该图像。我可以把它放回 SD 卡上并启动我的设备。所以步骤 1 和 2 没问题。
现在...
如果我完成了执行 `truncate --size=$[(6755394+1)*512] 'test.img' 的过程
和
GPT fdisk (gdisk) version 1.0.3
Warning! Disk size is smaller than the main header indicates! Loading
secondary header from the last sector of the disk! You should use 'v' to
verify disk integrity, and perhaps options on the experts' menu to repair
the disk.
Caution: invalid backup GPT header, but valid main header; regenerating
backup header from main header.
Warning! Error 25 reading partition table for CRC check!
Warning! One or more CRCs don't match. You should repair the disk!
Partition table scan:
MBR: protective
BSD: not present
APM: not present
GPT: damaged
****************************************************************************
Caution: Found protective or hybrid MBR and corrupt GPT. Using GPT, but disk
verification and recovery are STRONGLY recommended.
****************************************************************************
Disk test2.img: 6755395 sectors, 3.2 GiB
Sector size (logical): 512 bytes
Disk identifier (GUID): 159DCEDE-DBEA-4657-96D9-2CE178A96B7E
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 30310366
Partitions will be aligned on 64-sector boundaries
Total free space is 23555002 sectors (11.2 GiB)
Number Start (sector) End (sector) Size Code Name
1 64 8063 3.9 MiB 8300 loader1
2 8064 8191 64.0 KiB 8300 reserved1
3 8192 16383 4.0 MiB 8300 reserved2
4 16384 24575 4.0 MiB 8300 loader2
5 24576 32767 4.0 MiB 8300 atf
6 32768 262143 112.0 MiB 0700 boot
7 262144 6755394 3.1 GiB 8300 primary
显然 truncate（或 dd 就此而言）在 GPT 中表现不佳。在此文件上运行 gdisk 确认 gpt 已被丢弃
如果我在截断的文件上运行 gdisk，它会报告一个垃圾 GPT。
所以很明显我在这里遗漏了一些东西。显然，当磁盘文件的末尾发生更改时，GPT 会损坏或需要修复（不匹配）。即使它在第 2 步结束时很好/存在。我弄乱了 gdisk 并且无法修复它。除此之外，我想要一个可以用脚本做的解决方案，而这个 GPT 不起作用。
因此，因为它是 GPT，我是否需要使用截断或 dd 以外的其他东西，还是必须在截断后手动“修复”GPT。
这是 gdisk 验证报告的内容
Caution: The CRC for the backup partition table is invalid. This table may
be corrupt. This program will automatically create a new backup partition
table when you save your partitions.
Problem: The secondary header's self-pointer indicates that it doesn't reside
at the end of the disk. If you've added a disk to a RAID array, use the 'e'
option on the experts' menu to adjust the secondary header's and partition
table's locations.
Problem: Disk is too small to hold all the data!
(Disk size is 6755395 sectors, needs to be 30310400 sectors.)
The 'e' option on the experts' menu may fix this problem.
Problem: GPT claims the disk is larger than it is! (Claimed last usable
sector is 30310366, but backup header is at
30310399 and disk size is 6755395 sectors.
The 'e' option on the experts' menu will probably fix this problem
Partition(s) in the protective MBR are too big for the disk! Creating a
fresh protective or hybrid MBR is recommended.