我的分区表的第一个条目是：
$ sudo hexdump -Cv -n 16 -s 446 /dev/sda
000001be 80 01 01 00 83 fe ff ff 3f 00 00 00 81 1c 20 03 |........?..... .|
（-简历描述输出格式，-n 16请求 16 个字节，并且-s 446跳过前 446 个字节）
您可以看到我的第一个分区是主 Linux 分区，并且该分区从扇区 63 开始（例如，请参阅 [此处][1] 了解分区表的结构）。
然后我期望除了前 63 个扇区和其他分区之外，/dev/sda1 和 /dev/sda 完全相同。
但事实并非如此，/dev/sda1 的扇区#2 与 /dev/sda 的扇区#65 并不完全相同（但它们非常相似，只有 16 个字节不同）：
$ sudo hexdump -Cv -n 512 -s 65b /dev/sda
00008200 00 20 19 00 90 03 64 00 2d 00 05 00 5a 2f 56 00 |. ....d.-...Z/V.|
00008210 b6 b1 16 00 00 00 00 00 02 00 00 00 02 00 00 00 |................|
00008220 00 80 00 00 00 80 00 00 00 20 00 00 d8 38 ee 4c |......... ...8.L|
00008230 9a 01 ef 4c 05 00 24 00 53 ef 01 00 01 00 00 00 |...L..$.S.......|
00008240 59 23 e9 4c 00 4e ed 00 00 00 00 00 01 00 00 00 |Y#.L.N..........|
00008250 00 00 00 00 0b 00 00 00 00 01 00 00 3c 00 00 00 |............<...|
00008260 42 02 00 00 7b 00 00 00 85 23 eb f2 71 67 44 f5 |B...{....#..qgD.|
00008270 bb 8f 6f f2 3a 59 ff 4d 55 62 75 6e 74 75 00 00 |..o.:Y.MUbuntu..|
00008280 00 00 00 00 00 00 00 00 2f 75 62 75 6e 74 75 00 |......../ubuntu.|
00008290 d8 3c df 5d 00 88 ff ff 52 d0 ef 1d 00 00 00 00 |.<.]....R.......|
000082a0 c0 40 51 b6 00 88 ff ff 00 4e c8 bb 00 88 ff ff |[email protected]......|
000082b0 c0 f6 86 b8 00 88 ff ff 30 2e 0d a0 ff ff ff ff |........0.......|
000082c0 38 3d df 5d 00 88 ff ff 00 00 00 00 00 00 fe 03 |8=.]............|
000082d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000082e0 08 00 00 00 00 00 00 00 00 00 00 00 8a 53 d3 0e |.............S..|
000082f0 7c 7a 43 e4 8b fb ca e0 72 b7 fa c8 01 01 00 00 ||zC.....r.......|
00008300 00 00 00 00 00 00 00 00 16 4c 47 4b 0a f3 03 00 |.........LGK....|
00008310 04 00 00 00 00 00 00 00 00 00 00 00 fe 7f 00 00 |................|
00008320 24 b7 0c 00 fe 7f 00 00 01 00 00 00 22 37 0d 00 |$..........."7..|
00008330 ff 7f 00 00 01 00 00 00 23 37 0d 00 00 00 00 00 |........#7......|
00008340 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 08 |................|
00008350 00 00 00 00 00 00 00 00 00 00 00 00 1c 00 1c 00 |................|
00008360 01 00 00 00 e9 7f 00 00 00 00 00 00 00 00 00 00 |................|
00008370 00 00 00 00 04 00 00 00 9f 7d bb 00 00 00 00 00 |.........}......|
00008380 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00008390 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000083a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000083b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000083c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000083d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000083e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000083f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
相对
$ sudo hexdump -Cv -n 512 -s 2b /dev/sda1
00000400 00 20 19 00 90 03 64 00 2d 00 05 00 5a 2f 56 00 |. ....d.-...Z/V.|
00000410 b6 b1 16 00 00 00 00 00 02 00 00 00 02 00 00 00 |................|
00000420 00 80 00 00 00 80 00 00 00 20 00 00 df 76 ef 4c |......... ...v.L|
00000430 df 76 ef 4c 06 00 24 00 53 ef 01 00 01 00 00 00 |.v.L..$.S.......|
00000440 59 23 e9 4c 00 4e ed 00 00 00 00 00 01 00 00 00 |Y#.L.N..........|
00000450 0
答案1
很好的发现，因为我也能够在自己的系统上重现这种效果。在我的站点上，这种情况发生在 /dev/hda 上，因此这不是 SCSI 问题。
# uname -a
Linux X.X.X 2.6.26-2-686 #1 SMP Tue Mar 9 17:35:51 UTC 2010 i686 GNU/Linux
我认为whitequark这是一个缓存问题。以下是我对您的网站上发生的事情的解释（请注意，我不确定我的解释是否正确)：
/dev/sda1 正在使用中。因此“sync”会在每次刷新日志（或类似操作）时更新超级块。因此磁盘 /dev/sda1 已更改。
但是内核不使用 /dev/sda 和 /dev/sda1 的组合缓存，而是两个“文件”单独缓存。因此，更新 /dev/sda1（同步）不会使 /dev/sda 的缓存失效。因此，从 /dev/sda 读取会显示旧缓存值（因此缓存与硬盘不同步），而 /dev/sda1 会显示正确的（新）值。
以下是我这边看到的情况。我之前在 /dev/hda 上做过一些转储，所以它已经缓存了一些旧数据：
# od -tx1z -N 10k /dev/hda2 > NOW1
# dd ibs=512 skip=1975995 if=/dev/hda | od -tx1z -N 10k > MAIN1
# diff NOW1 MAIN1
3,4c3,4
< 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 be 1a 39 00 >..1.W.a..}....9.<
< 0002020 4e ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >N.!.............<
---
> 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 4e 1b 39 00 >..1.W.a..}..N.9.<
> 0002020 52 ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >R.!.............<
# od -tx1z -N 10k /dev/hda2 > NOW1
# diff NOW1 MAIN1
3,4c3,4
< 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 be 1a 39 00 >..1.W.a..}....9.<
< 0002020 4e ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >N.!.............<
---
> 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 4e 1b 39 00 >..1.W.a..}..N.9.<
> 0002020 52 ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >R.!.............<
# od -tx1z -N 10k /dev/hda2 > NOW2
# dd ibs=512 skip=1975995 if=/dev/hda | od -tx1z -N 10k > MAIN2
# diff MAIN1 MAIN2
# diff NOW2 MAIN2
3,4c3,4
< 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 f0 19 39 00 >..1.W.a..}....9.<
< 0002020 41 ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >A.!.............<
---
> 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 4e 1b 39 00 >..1.W.a..}..N.9.<
> 0002020 52 ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >R.!.............<
106c106
< 0012440 00 80 14 00 01 80 14 00 02 80 14 00 00 00 01 00 >................<
---
> 0012440 00 80 14 00 01 80 14 00 02 80 14 00 00 00 00 00 >................<
334c334
< 0021540 00 80 4d 00 01 80 4d 00 02 80 4d 00 02 00 63 3e >..M...M...M...c><
---
> 0021540 00 80 4d 00 01 80 4d 00 02 80 4d 00 02 00 64 3e >..M...M...M...d><
/dev/hda 没有显示任何更新，而 /dev/hda2 显示了一些变化。但是当我刷新缓存并重试时，所有内容都显示相同：
# hdparm -f /dev/hda
/dev/hda:
# dd ibs=512 skip=1975995 if=/dev/hda | od -tx1z -N 10k > MAIN2
# diff MAIN1 MAIN2
3,4c3,4
< 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 4e 1b 39 00 >..1.W.a..}..N.9.<
< 0002020 52 ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >R.!.............<
---
> 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 dc 1a 39 00 >..1.W.a..}....9.<
> 0002020 96 ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >..!.............<
36,37c36,37
< 0010300 00 00 03 00 01 00 03 00 02 00 03 00 00 00 bb 3b >...............;<
< 0010320 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >................<
---
> 0010300 00 00 03 00 01 00 03 00 02 00 03 00 00 00 bc 3b >...............;<
> 0010320 7f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >................<
48c48
< 0010600 00 00 06 00 01 00 06 00 02 00 06 00 03 00 18 3f >...............?<
---
> 0010600 00 00 06 00 01 00 06 00 02 00 06 00 04 00 18 3f >...............?<
106c106
< 0012440 00 80 14 00 01 80 14 00 02 80 14 00 00 00 00 00 >................<
---
> 0012440 00 80 14 00 01 80 14 00 02 80 14 00 00 00 01 00 >................<
# od -tx1z -N 10k /dev/hda2 > NOW2
# diff NOW2 MAIN2
# diff MAIN1 MAIN2
3,4c3,4
< 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 4e 1b 39 00 >..1.W.a..}..N.9.<
< 0002020 52 ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >R.!.............<
---
> 0002000 00 00 31 01 57 c4 61 02 04 7d 1e 00 dc 1a 39 00 >..1.W.a..}....9.<
> 0002020 96 ea 21 01 00 00 00 00 02 00 00 00 02 00 00 00 >..!.............<
36,37c36,37
< 0010300 00 00 03 00 01 00 03 00 02 00 03 00 00 00 bb 3b >...............;<
< 0010320 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >................<
---
> 0010300 00 00 03 00 01 00 03 00 02 00 03 00 00 00 bc 3b >...............;<
> 0010320 7f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >................<
48c48
< 0010600 00 00 06 00 01 00 06 00 02 00 06 00 03 00 18 3f >...............?<
---
> 0010600 00 00 06 00 01 00 06 00 02 00 06 00 04 00 18 3f >...............?<
106c106
< 0012440 00 80 14 00 01 80 14 00 02 80 14 00 00 00 00 00 >................<
---
> 0012440 00 80 14 00 01 80 14 00 02 80 14 00 00 00 01 00 >................<
关于如何重现的简短说明：
- 确保您的系统处于空闲状态并且具有足够的 RAM 用于缓存。
- 运行
fdisk -u -l以查找分区的起始位置。在我这边是 1975995 - 选择已安装的分区和完整的驱动器。
- 然后执行两个转储（NOW1、MAIN1）并进行比较。它们应相等。
- 对分区稍作修改，同步。
- 留出更多时间。
- 稍加改变，再次同步。
- 转储 NOW2，它应与 NOW1 不同
- 转储 MAIN2，它应该不是与 MAIN1 不同！
- 执行
hdparm你的驱动器的行 - 再次转储 MAIN2，现在它将与 MAIN1 不同。
- 如果你足够快，NOW2 和 MAIN2 将会相等。
答案2
进行类似测试后，我没有发现任何差异。可能是在每次转储之间写入了特定扇区。
以下命令比较从 /dev/sda 和 /dev/sda1 提取的同一扇区的第一个~48MB：
$ diff <(sudo hexdump -Cv -n $((512*100000)) -s 0x7e00 /dev/sda | awk '{$1=""}1' ) <(sudo hexdump -Cv -n $((512*100000)) /dev/sda1 | awk '{$1=""}1' )
其中 0x7e00 是第一个分区的偏移量。