My personal wishlist

Nobody III hungryninja101 at
Tue Apr 2 20:00:28 CEST 2019

I suppose we could use this mechanism, which we already use for ioctl, for
several other file attributes as well, e.g. timestamps. We could put it all
under a directory named .attrs-(filename).

As for performance, the filesystem drivers could implement attribute access
as separate files while still using attrs/xattrs as the underlying storage
mechanism. The main issue with this is when attributes are copied before
the files. However, if we make the attributes transparent in libc, this
shouldn't be much of an issue, especially if we guarantee that the
attributes come after the files in the dirent listing. Overall, I think
this could be a relatively clean way to implement both regular and extended
attributes. Any other thoughts on this?

On Tue, Apr 2, 2019, 7:54 AM Valery V. Sedletski via users <
users at> wrote:

> Norman Feske wrote:
> > Hello Valery,
> >
> > thanks for the background info about extended attributes.
> >
> >> mandatory to be existed when opening the file). So, EA's on FAT32 are
> >> laying around near the file itself, in the same directory, in a hidden
> >> file. The disadvantage is that EA files are laying around everywhere,
> >> like a trash. So, if a file system doesn't support EA's, they can be
> >> emulated like this.
> > I admittedly have not much clue about file systems. To me as a user,
> > features like this (this also goes for the strange "resource forks" of
> > old Mac OS) look like glorified directories. They seem to be just more
> > complicated, less transparent, and cause friction when moving data
> > across file systems. Given my poor background, please take the following
> > with a grain of salt.
> Yes, also WinNT goes even further: it supports files having multiple
> streams of data. Resource fork from MacOS or EA can be implemented
> as streams too. Only Resource fork stores multiple metainfo in one
> stream, and EA's can be implemented both ways: as a single stream,
> or multiple streams.
> > I wonder, in situations where meta data has to contain a lot of domain
> > knowledge, wouldn't sqlite be a good way to organize it?
> Probably yes, though, sqlite files are just tables with fixed number
> of fields. Storing EA in SQLite or any other relational database should
> require a data schema with multiple tables, which may be too complex,
> because a file can have any number of EA's, whereas a table contains
> a fixed number of fields. So, it may require a complicated data schema.
> > In situations where meta data are simple annotations to files (like
> > storing the icon for a file), why can't both the file and the meta data
> > go to into a directory? This is the way Risc OS (and also today's Mac
> > OS, AFAIK) deal with the situation.
> They could be not simple annotations/text data, but they also could
> be any small binary data attached to a file (icons being a good example).
> Directory could be too small to fit all EA's. Usually, if EA's are small
> enough, file systems like HPFS or JFS can store them in inode/fnode
> inline. But if they are not fit into inode, they are put into separate
> streams of sectors. Directory usually only contains a reference to EA's
> (a first sector/block/cluster number of EA stream, or a handle to a
> particular EA, or just some bits which denote that EA is present.).
> Not sure about RISCOS, but AFAIK, MacOS uses a separate stream
> of data to store its resource forks. Directories ideally should only
> contain a mapping between inode number and a file name. All standard
> metadata goes into an inode. But if it's not fit into inode, it goes into
> separate stream. Inode size is usually limited.
> > Hence, there is currently no plan to extend Genode's file-system session
> > with extended attributes.
> Yes, I of course, understand the wish to keep the interfaces and
> architecture simpler.
> > Valery's remark about an emulation scheme looks good to bridge the gap.
> > This way, such a feature can be implemented within the libc where the
> > libc would rely on some conventions. E.g., when asked for the extended
> > attributes for a certain 'file', the libc may - under the hood - read
> > the meta data from a file called '.xa/file'. Yes, this would spill the
> > '.xa' directories here and there but it leaves the complexity out of
> > base mechanisms like the VFS or the file-system session.
> Yes, the EA/xattr store can be moved to a separate subdirectories
> of each current subdirectory (like in svn working copies.). This
> seems to be a better idea than to have EA's laying around along
> with their files. So, they will not pollute the usual disk contents.
> Yes, it looks be possible to have EA support on a libc level.
> Usually, EA's are returned along with standard file metainfo
> when setting or retrieving FileInfo/PathInfo or searching files.
> So, e.g., a file search retrieves a set of records, per each file. These
> records may contain a standard metainfo (inode contents), as well
> as EA contents. Also, when opening files, "open" syscall could create
> EA's for that file. Pointer to EA data is specified as a separate syscall
> parameter. Though, the open/fopen calls don't contain such
> parameters, but a native syscall does. Creating a FS-independent
> wrapper on libc level looks possible, though I fear that it could
> be inefficient and slow. In a file search example, if a FS returned
> a set of records containing file metadata, then libc wrapper should
> process this set of records, and add EA data to each record. This
> looks to be working, but it could be slow. Usually, a FS does the same
> with a single step: usually both, metadata contained in inode, and EA's,
> a returned at once. For speeding things up, the FS doesn't check
> each file for EA presence, instead, a flag is used, which specifies,
> whether file has EA's. If not, the EA lookup is just omitted.
> WBR,
> valery
>  > Cheers
>  > Norman
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