On 25/05/2022 18:05, Goffredo Baroncelli wrote:
Hi All,
recently I discovered that BTRFS allow to reflink a file only if the flag
FS_NOCOW_FL is the same on both source and destination.
In the end of this email I added a patch to "cp" to set the FS_NOCOW_FL flag
according to the source.
Even tough this works, I am wondering if this is the expected/the least
surprise behavior by/for any user. This is the reason why this email is tagged
as RFC.
Without reflink, the default behavior is that the new file has the FS_NOCOW_FL
flag set according to the parent directory; with this patch the flag would be
the same as the source.
I am not sure that this is the correct behviour without warning the user of
this change.
Another possibility, is to flip the NOCOW flag only if --reflink=always is
passed.
Thoughts ?
This flag corresponds to the 'C' chattr attribute,
to allow users to explicitly disable CoW on certain files
or files within certain dirs.
I don't think cp should be overriding that explicit config. I.e.:
cp --reflink=auto => try reflink but fall back to normal copy
cp --reflink=always => try reflink and fail if not possible
We would need another option to bypass system config
(like --reflink=force), however I don't think that's
appropriate functionality for cp.
thanks,
Pádraig
