Quoting Johannes Schauer Marin Rodrigues (2024-05-04 16:32:50) > I originally observed this issue when creating ext4 disk images on a 9p > filesystem which differed from the images I created on a tmpfs. I observed > that the difference also exists when the underlying file system is fat32, so > I'm using this as an example here. For what it's worth, the ext4 filesystem > images created on a tmpfs are identical to those created on an ext4 fs. To > demonstrate the issue, please see the script at the end of this mail (it > requires sudo to mount and unmount the fat32 disk image). As you can see from > the printed hashes, the disk images produced outside the fat32 disk are > always identical as expected. The diff between the reproducible images and > those stored on fat32 is also very short but I don't know what data is stored > at those points: > > @@ -85,7 +85,7 @@ > 00000540: 0000 0000 0000 0000 0000 0000 0000 1000 ................ > 00000550: 0000 0000 0000 0000 0000 0000 2000 2000 ............ . . > 00000560: 0200 0000 0000 0000 0000 0000 0000 0000 ................ > -00000570: 0000 0000 0401 0000 8c04 0000 0000 0000 ................ > +00000570: 0000 0000 0401 0000 4900 0000 0000 0000 ........I....... > 00000580: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > 00000590: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > 000005a0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > @@ -125,9 +125,9 @@ > 000007c0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > 000007d0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > 000007e0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > -000007f0: 0000 0000 0000 0000 0000 0000 264c 0251 ............&L.Q > +000007f0: 0000 0000 0000 0000 0000 0000 64ca bba5 ............d... > 00000800: 1200 0000 2200 0000 3200 0000 9d03 7300 ...."...2.....s. > -00000810: 0200 0000 0000 0000 babb 8a41 7300 2004 ...........As. . > +00000810: 0200 0400 0000 0000 babb 8a41 7300 69f5 ...........As.i. > 00000820: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > 00000830: 0000 0000 0000 0000 bc7a 6e31 0000 0000 .........zn1.... > 00000840: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > > Any idea what is going on? Is there a better way to diff two ext4 disk images > than diffing the xxd output? If I try diffing the dumpe2fs output I get these > differences: > > @@ -32,7 +32,7 @@ > Maximum mount count: -1 > Last checked: Fri May 3 16:14:49 2024 > Check interval: 0 (<none>) > -Lifetime writes: 1164 kB > +Lifetime writes: 73 kB > Reserved blocks uid: 0 (user root) > Reserved blocks gid: 0 (group root) > First inode: 11 > @@ -44,7 +44,7 @@ > Directory Hash Seed: 0b7f9cfd-0113-486c-a453-4f5483bd486b > Journal backup: inode blocks > Checksum type: crc32c > -Checksum: 0x51024c26 > +Checksum: 0xa5bbca64 > Checksum seed: 0xf81d767d > Orphan file inode: 12 > Journal features: (none) > @@ -56,7 +56,7 @@ > Journal start: 0 > > > -Group 0: (Blocks 1-2047) csum 0x0420 > +Group 0: (Blocks 1-2047) csum 0xf569 [ITABLE_ZEROED] > Primary superblock at 1, Group descriptors at 2-2 > Reserved GDT blocks at 3-17 > Block bitmap at 18 (+17), csum 0x7abcbbba > > Why would these bits differ depending on the filesystem on which the disk image > is stored? Is there a way to equalize this information so that the disk image > looks the same independent on the underlying filesystem? The diff becomes a bit smaller when using -E lazy_itable_init=0,assume_storage_prezeroed=0,nodiscard @@ -85,7 +85,7 @@ 00000540: 0000 0000 0000 0000 0000 0000 0000 1000 ................ 00000550: 0000 0000 0000 0000 0000 0000 2000 2000 ............ . . 00000560: 0200 0000 0000 0000 0000 0000 0000 0000 ................ -00000570: 0000 0000 0401 0000 ac04 0000 0000 0000 ................ +00000570: 0000 0000 0401 0000 4900 0000 0000 0000 ........I....... 00000580: 0000 0000 0000 0000 0000 0000 0000 0000 ................ 00000590: 0000 0000 0000 0000 0000 0000 0000 0000 ................ 000005a0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ @@ -125,7 +125,7 @@ 000007c0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ 000007d0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ 000007e0: 0000 0000 0000 0000 0000 0000 0000 0000 ................ -000007f0: 0000 0000 0000 0000 0000 0000 fb8d a90f ................ +000007f0: 0000 0000 0000 0000 0000 0000 64ca bba5 ............d... 00000800: 1200 0000 2200 0000 3200 0000 9d03 7300 ...."...2.....s. 00000810: 0200 0400 0000 0000 babb 8a41 7300 69f5 ...........As.i. 00000820: 0000 0000 0000 0000 0000 0000 0000 0000 ................ @@ -32,7 +32,7 @@ Maximum mount count: -1 Last checked: Fri May 3 16:14:49 2024 Check interval: 0 (<none>) -Lifetime writes: 1196 kB +Lifetime writes: 73 kB Reserved blocks uid: 0 (user root) Reserved blocks gid: 0 (group root) First inode: 11 @@ -44,7 +44,7 @@ Directory Hash Seed: 0b7f9cfd-0113-486c-a453-4f5483bd486b Journal backup: inode blocks Checksum type: crc32c -Checksum: 0x0fa98dfb +Checksum: 0xa5bbca64 Checksum seed: 0xf81d767d Orphan file inode: 12 Journal features: (none) The "Lifetime writes" being much higher on fat32 suggests that despite "nodiscard", less zeroes were written out when ext4 or tmpfs are the underlying FS? Thanks! cheers, josch
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