Re: file-backed cgd backup question

[mlelstv%serpens.de@localhost (Michael van Elst)]

gdt%lexort.com@localhost (Greg Troxel) writes:

>I dimly knew this, but keep forgetting.  Reading vndconfig(8), it does
>not explain that the normal path leads to incorrect behavior (stale
>reads from file cache even after closing the vnd, mtime).

vnd opens the backing file when the unit is created and closes
the backing file when the unit is destroyed. Then you can access
the file again.

The data is written directly to the allocated blocks of the file.
So exclusively opening  the backing file _or_ the vnd unit should
also be safe. But that's not much different from accessing any file
concurrently, which also leads to "corrupt", inconsistent backups.

Updating the backing file mtime on close sounds useful. I'm not sure
what effect updating atime/mtime on every access would have.

> This
>optimization is sufficiently dangerous and not expected that it needs to
>be documented clearly and loudly.  I just added a note to the man page.

I think the reference to "ciphertext" should be adjusted and the
text should be toned more neutral when describing the functionality.

Pointing to the -i option to disable the optimization unconditionally
might also be helpful.