Re: Can BTRFS handle XATTRs larger than 4K?

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

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On 2014-12-22 15:04, Richard Sharpe wrote:
> On Mon, Dec 22, 2014 at 10:09 AM, Austin S Hemmelgarn
> <ahferroin7@gmail.com> wrote:
>> On 2014-12-22 12:27, Richard Sharpe wrote:
>>>
>>> On Mon, Dec 22, 2014 at 6:28 AM, Austin S Hemmelgarn
>>> <ahferroin7@gmail.com> wrote:
>>>>
>>>> On 2014-12-19 21:07, Richard Sharpe wrote:
>>>>>
>>>>>
>>>>> Hi folks,
>>>>>
>>>>> I need a Linux file system that supports XATTRs up to 64K.
>>>>>
>>>>> Can BTRFS support that or is XFS the only Linux file system with such
>>>>> support?
>>>>>
>>>> At the moment, BTRFS is limited to xattrs that fit inline in the metadata
>>>> nodes (so ~3900 bytes for a 4k leafsize).
>>>>
>>>> XFS, however, isn't the only Linux filesystem that supports xattrs that
>>>> size.  Assuming that you are using a recent kernel, you can also use such
>>>> xattrs on at least:
>>>>    * XFS
>>>>    * JFS
>>>>    * ext4
>>>>    * reiserfs (I think, not 100% certain about this one though)
>>>>    * OCFS2 (even though it is technically a cluster fs, it can be run
>>>> single
>>>> node without the clustering)
>>>>    * ZFS (IIRC, ZFS supports unlimited xattr size)
>>>>    * NTFS (no limit on xattr size, though you should use NTFS-3G instead
>>>> of
>>>> the in-kernel driver)
>>>>    * SquashFS (read-only)
>>>>    * HFS+ (Also no limit on xattr size)
>>>> Of these, I'd personally suggest using XFS unless you need to be able to
>>>> shrink the filesystem, in which case I'd suggest ext4.
>>>
>>>
>>> Thanks for the info. I hadn't realized that ext4 had lifted the
>>> restriction.
>>>
>> Yeah, it would be nice if there was more clarity in the documentation.
>>
>> 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.
>
> Well, there is a big difference in the semantics of Alternate Data
> Streams (ADSs) and XATTRs.
>
> For example, you can seek on an ADS and read at any offset. You cannot
> do that on an XATTR (at least, not with the semantics provided by the
> UNIX interface.)
>
I'm not trying to say the semantics are identical, just that the 
functionality is pretty similar.  A better way to put it would be that 
xattrs are an extremely limited form of ADS/Forks.


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