DT_WHT (Re: [PATCH 00/13] overlay filesystem: request for inclusion (v16))

From: J. R. Okajima
Date: Thu Mar 14 2013 - 08:58:07 EST

Miklos Szeredi:
> On Thu, Mar 14, 2013 at 12:19 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > As for whiteouts... I think we ought to pull these bits of unionmoun
> > queue into the common stem and add the missing filesystems to them;
> > ext* and ufs are trivial (keep in mind that FFS derivatives, including
> > ext*, have d_type in directory entry and type 14 (DT_WHT) is there
> > precisely for that purpose). btrfs also has "dir_type" thing - 8bit
> > field...
> What about userspace interfaces? Are we allowed to extend d_type and
> st_mode without breaking things?

Introducing a new d_type value can be a headache for other filesystems
or other OSs. Whiteout and opaque by xattr is an idea, but I prefer more
primitive way (special hidden filename) since xattr may force users to
change their configuration and it consumes disk space a little.
Filesystems may have a limit for the consumed size by xattr. If xattr
reaches the limit, then several operation in overlayfs will be unusable.

J. R. Okajima
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