[Bug 242341] GEOM / GEOM_PART: silent discard MBR modification

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=242341

--- Comment #5 from Andrey V. Elsukov <ae at FreeBSD.org> ---
(In reply to Tomasz "CeDeROM" CEDRO from comment #3)

> 1. I would like to clarify why I was unable to write to MBR (first 512 bytes) of the da1 while there was no problem with writhing to the rest of the disk?

How did you know that? Did you compare the first 512 bytes on both disks after
copying?

> 2. GEOM_PART: integrity check failed (da1, MBR) <- could that block / discard writes to MBR silently?

This usually means that parition table's metadata is wrong. If it is wrong, and
kern.geom.part.check_integrity is set, then GEOM_PART just ignores the
partition table.

> 3. Why `dd if=/dev/da0 of=/dev/da1` did copy all disk except the MBR?

How did you know that? Did you compare the first 512 bytes on both disks after
copying? I assume you have disks with different size and GEOM_PART just rejects
MBR on the copied disk, because its integrity check has failed. 

> 4. Why I was unable to write to MBR from a file but it was possible to write that data from a file over md0?

Again, how did you know that?

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