Re: Inquiry: Possible built-in support for longer filenames in ext4 (beyond 256 bytes)

["Theodore Tso" <tytso@mit.edu>]

On Fri, Dec 12, 2025 at 10:10:36AM -0800, Andi Kleen wrote:
> 
> Perhaps you could use xattrs on the directory inode to store the longer
> names, or the overflow.
> 
> One problem is that they may need to be big, exceeding xattr
> limits, but perhaps some total limit on the longer file names
> would be acceptable.

With ext4, there is a limit of a single file system block for all
extended attributes.  You can store the value of extended attribute in
an inode, in which case you only have the four byte inode number in
the xattr block.  But still, if you assume 16 bytes of overhead for
each xattr entry, plus the xattr header, there's only room for 9 400
byte directory entries.

And you wouldn't want to have a lot of directory entries stored in
xattrs anyway, since searching them would have to be a brute force,
O(n) search.  You wouldn't be able to use a hash tree for fast
lookups.

So even if you didn't have the xattr limits, if you had a large number
of very long file names stored in xattrs, it would be a performance
disaster.

Cheers,

						- Ted