Re: Questions about FIEMAP

[Duncan <1i5t5.duncan@cox.net>]

Wang, Zhiye posted on Wed, 28 Oct 2015 09:57:29 +0000 as excerpted:

> Thank you all for your comments.
> 
> A further question is: if I mount a btrfs file system in "readonly"
> mode, will any operation cause the blocks of a file get changed?

Note that both bind-mounts and btrfs subvolume mounts can be used to make 
parts of a filesystem appear in multiple locations in the filesystem 
tree.  Because these different mounts can be separately mounted read-only 
or writable, there's no guarantee that just because a filesystem or part 
of it is read-only mounted in one location, it's read-only mounted 
everywhere it can be accessed, and thus no guarantee that files even on a 
read-only mounted filesystem or subvolume won't actually change out from 
under you.

However, bind-mounts in particular aren't btrfs specific, so just because 
btrfs subvolumes add another case in which the above can be true, doesn't 
mean bind-mounts can't be used on other filesystems to effect the same 
sort of otherwise read-only file instabilities.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman