Re: file-backed cgd backup question

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

gdt%lexort.com@localhost (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.