Streams support in Linux

[Steve French]

On Mon, Aug 27, 2018 at 7:45 PM Theodore Y. Ts'o <tytso at mit.edu> wrote:
>
> On Mon, Aug 27, 2018 at 11:21:43AM -0700, Matthew Wilcox wrote:
> > On Mon, Aug 27, 2018 at 10:05:31AM -0700, Jeremy Allison wrote:
> > > I can't think of a *single* case where a stream adds more
> > > utility than an EA used in the same case.
> > >
> > > I don't want theoretical "well it would be nice if..",
> > > I want clear "we couldn't have done it any other way"
> > > kinds of things.
> >
> > I started this thread with such an example.  The fs-verity patch proposed
> > wants to store hundreds of megabytes of data associated with a particular
> > file.  The current solution is to append it to the end of the data then
> > magic to set i_size lower but not remove the data from the file like a
> > truncate would.  Then more magic to read that data.
>
> Sure, but so what?  The Merkle Tree only needs to be read by the
> kernel, and we only need to install the fs-verity data once.  And
> putting it at the end of the file works just fine.  Theoretically we
> could do it other ways, but this is almost certainly easier and
> cleaner than if we had to open a streams file from kernel code.
>
> Doing it using streams would actually be *harder* and is only of use
> for people who like more structured designs.  But to be clear, this is
> *not* something I'm asking for as one of the original designers of
> fs-verity.

Given that streams need to be read to backup Macs and Windows
(and for a few features of these servers mentioned earlier)
and would be exposed in ntfs (and SMB3 remotely) locally on Linux,
seems useful to me to have some consistent way to open and read
them on Linux even if we don't want to generalize it to other local fs.
The protocol supports it fine (just a file with a reserved character
':' in it) but a little tricky to avoid name conflict with posix ':'
in filenames


-- 
Thanks,

Steve