g...@lexort.com (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.
