g...@lexort.com (Greg Troxel) writes: >> 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.
>Is there a guarantee of cache consistency for writes before and reads >after? Before the unit is created you can access the file and after the unit is destroyed you can access the file. That's always safe. I also think that when the unit is configured but not opened (by device access or mounts) it is safe to access the file. >> 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. >That's a different kind of corrupt. Yes, but in the end it's the same, the "backup" isn't usuable. You cannot access the backing file to get a consistent state of the data while a unit is in use. And that's independent of how vnd accesses the bits. N.B. if you want to talk about dangers, think about fdiscard(). I doubt that it is safe in the context of the vnd optimization.
