Displaying streams as xattrs

Andrew Walker awalker at ixsystems.com
Tue May 23 02:23:32 UTC 2023

On Mon, May 22, 2023 at 8:00 PM ronnie sahlberg via samba-technical
<samba-technical at lists.samba.org> wrote:
> Yeah, I don't think we should surface these as xattrs.
> Xattrs are already way too small for most of the usecases of ADS on
> windows (file metadata for explorer etc)
> and they are also just an atomic blob and not a proper filedescriptor.
> Since ADS is still just a file, any application that in the future
> will use ADS features should only do so via
> a proper filedescriptors, where it is possible to
> seek/read/write/truncate/...  so IMHO we shouldn't offer them an
> alternative but inferior API. Because they might mistakenly start to use it. :-(
> There are no real applications, yet, for linux that uses ADS but there
> are many that potentially could use ADS or
> become ADS aware. GUI Filebrowsers would be a nice usecase. As would
> making 'cp', 'mv', 'tar', 'rsync', ... ADS aware.
> So let's not do it with xattrs.
> No one needs/asks for this right now so it would be code we will have
> to maintain but has no users.
> What we should do though is think about and talk with the NTFS folks
> so that we make sure our aims and APIs will align with the plans they
> have.
> And once we have multiple filesystems supporting it (cifs.ko and ntfs)
> then we can start thinking about how to encourage external users and
> applications to use it.
> There are really nice use-cases for ADS where one can store additional
> metadata within the "file" itself.
> regards
> ronnie s
> On Tue, 23 May 2023 at 02:21, Jeremy Allison <jra at samba.org> wrote:
> >
> > On Mon, May 22, 2023 at 01:39:50AM -0500, Steve French wrote:
> > >On Sun, May 21, 2023 at 11:33 PM ronnie sahlberg
> > ><ronniesahlberg at gmail.com> wrote:
> > >>
> > >> A problem  we have with xattrs today is that we use EAs and these are
> > >> case insensitive.
> > >> Even worse I think windows may also convert the names to uppercase :-(
> > >> And there is no way to change it in the registry :-(
> > >
> > >But for alternate data streams if we allowed them to be retrieved via xattrs,
> > >would case sensitivity matter?  Alternate data streams IIRC are already
> > >case preserving.   Presumably the more common case is for a Linux user
> > >to read (or backup) an existing alternate data stream (which are usually
> > >created by Windows so case sensitivity would not be relevant).
> >
> > Warning Will Robinson ! Mixing ADS and xattrs on the client side is a receipe for
> > confusion and disaster IMHO.
> >
> > They really are different things. No good will come of trying to mix
> > the two into one client namespace.

Solaris / Illumos had a neat feature where you could say openat(fd,
O_RDWR | O_XATTR) to open a stream on ZFS and then do normal file IO
to the stream (pread, pwrite, etc).

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