Re: Can BTRFS handle XATTRs larger than 4K?

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2014-12-22 15:06, Richard Sharpe wrote:
> On Mon, Dec 22, 2014 at 10:43 AM, Chris Murphy <lists@colorremedies.com> wrote:
>> On Mon, Dec 22, 2014 at 11:09 AM, Austin S Hemmelgarn
>> <ahferroin7@gmail.com> wrote:
>>
>>> Personally, I'd love to see unlimited length xattr's like NTFS and HFS+ do,
>>> as that would greatly improve interoperability (both Windows and OS X use
>>> xattrs, although they call them 'alternative data streams' and 'forks'
>>> respectively), and provide a higher likelihood that xattrs would start
>>> getting used more.
>>
>> This is two years old, but it looks like NFS will not support xattr.
>> http://comments.gmane.org/gmane.linux.nfs/53259
>>
>> It looks like SMB does support xattr (and sometimes requires it) but I
>> have no idea to what degree, including the host/client preservation on
>> different filesystems. [1] It would still be helpful for cp and rsync
>> to be able to preserve xattr, however Apple has moved to a new on-disk
>> format that makes the future of reading OS X volumes on Linux an open
>> question. [2]
>
> Those are the old OS 2 XATTRs, better known as EAs, and NTFS says that
> you can support EAs or you can have Reparse Points, but not both
> (basically, they re-used the EA Length field as the reparse tag).
> Also, Windows (of any flavor) does not make it easy to access EAs.
>
But OS/2 style XATTRS are not the same as NTFS Alternative Data Streams, 
which technically (because of Windows backward compatibility interfaces) 
don't need a huge amount of support from SMB.  They were originally 
added to support SFM in NT3.1, so that windows could store resource 
forks.  The two primary uses on windows today are the file history 
interface in Win8/8.1 and the 'zone_identifier' saved with downloads by 
most modern browsers.  They're actually pretty easy to get to, you just 
append the ADS name to the end of the filename with a : separating them, 
and you can access it like a regular file (which is part of why : isn't 
a legal character in a windows filename).  Most people don't know about 
them because they don't get listed in windows explorer, even with hidden 
files and protected OS file visible.  The actual on-disk format for them 
is actually kind of interesting, the file data itself (what in the Apple 
world is called the Data Fork) is actually stored as an unnamed ADS 
associated with the filename.


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