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Old 7th December 2017
gustaf gustaf is offline
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Default Verify integrity of disk image file written to disk ?

I have copied the file https://<local-mirror-domain-name>/pub/OpenBSD/6.2/amd64/install62.fs
to a flash drive using dd(1), according to the instrucions in /pub/OpenBSD/6.2/amd64/INSTALL.amd64.

These instructions also suggest that
...copying the image back to a file and doing a compare or checksum is a good way to verify that the USB key is readable and free of read/write errors.
This would be relatively easy to accomplish with a reverse dd(1) if the file and the drive were the same size: $ doas dd if=/dev/rsd1c of=/path/to/install62.fs.copy bs=1m.

The problem is that /dev/rsd1c (the drive that contains the copied image) is larger than install62.fs (the original file). Any attempt to copy and compare will inevitably fail.

One possible solution is to extract the 'install62.fs' data from the drive before copying it to a file, but I can't find a command to do this.

Once I have a duplicate file, it's a simple matter of running cmp(1), sha256(1), or sha512(1) to see if the two files match.

Does anyone have any suggestions for how to copy the image on the drive back to a file so I can verify that the drive was written correctly?

I can mount the file systems and see the files, so I think it's OK, but I'd like to have some extra assurance since this is part of my emergency recovery plan.

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Old 7th December 2017
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jggimi jggimi is offline
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dd(1) has a count=n option.

Example: The cd62.iso file for the amd64 -release is 9953280 bytes. This is not divisible by megabytes, but it is divisible by 8K (8192 bytes). There are 1215 8K blocks in the file. I used a 2GB vnd(4) test device, creating the 2GB sparse file that backed the device with vmctl(8) as it was convenient, as I don't have a spare USB stick at the moment.

Writing to the drive with 1MB blocks, dd() completed 9 full 1MB blocks, plus 1 partial block:
$ sha256 cd62.iso 
SHA256 (cd62.iso) = e66b406cf5775c934b04eb2c3af1f8f6f4704f67445b2dd22916a7b962a74667
$ vmctl create test.file -s 2g
vmctl: imagefile created
$ doas vnconfig vnd0 test.file 
$ doas dd if=cd62.iso of=/dev/rvnd0c bs=1m
9+1 records in
9+1 records out
9953280 bytes transferred in 0.149 secs (66761325 bytes/sec)
Testing the resulting data written to the drive, I used a 1215 count with an 8K block size. The sha256(1) hash was identical.
$ doas dd if=/dev/rvnd0c bs=8192 count=1215 | sha256
1215+0 records in
1215+0 records out
9953280 bytes transferred in 0.340 secs (29257170 bytes/sec)

Last edited by jggimi; 7th December 2017 at 11:57 AM. Reason: typos
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Old 14th December 2017
gustaf gustaf is offline
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Hi jggimi,

Thanks for the example. I missed the count=n option when I read dd(1).

It turns out that this drive wasn't actually written correctly, in spite of the fact that it appeared OK on visual inspection.

The initial sha256(1) checksum comparison failed. Running cmp(1) revealed 40 errors.

I re-wrote the drive and now the checksums match.

Lesson: Appearances are deceiving. If byte-for-byte accuracy is important, run a checksum.
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