Re: Can't cp --reflink files on a Ext4-converted FS w/o checksums

[Roman Mamedov <rm@romanrm.net>]

On Wed, 26 Nov 2014 16:00:23 -0800
Robert White <rwhite@pobox.com> wrote:

> Uh... you may _still_ have no checksums on any of those data extents. 
> They are not going to come back until you write them to a normal file 
> with a normal copy. So you may be lacking most of the data validation 
> features of this filesystem.

Well, this FS is coming from being Ext4 for years, so it's not worse off now
than it was before. And anyways the main feature that I wanted were snapshots.

> You might want to go experiment. Make another new subvol (or at least a 
> directory in a directory/root/subvol that never had the +C attribute 
> set) and see if you can cp --reflink any of these files into that 
> subdirectory without repeating the +C trick.

Ha, indeed I can't. Maybe there should be a way to generate checksums without
rewriting files, just via reading them, then calculating and writing checksum
to metadata.

> Clearing NODATACOW does _not_ clear NODATASUM (at least not on a 
> non-empty file) as near as I can tell, so that directory hierarchy and 
> its subsequent snapshots is likely "less safe" than you think.

The nodatasum flag also isn't accessible via chattr, is it?

-- 
With respect,
Roman