[ACL-Devel] FS meta information (properties/attributes/...)

[Andreas Gruenbacher]

Curtis Anderson wrote:
> [...]
> IMHO, the directory-like approach is a slippery slope leading to not having
> extended attributes at all.  As soon as you go that route, people want access
> to the timestamps on each "fork", they want separate permisions on each "fork",
> and they want attribute values greater than 4GB.  They should just have coded
> their app to use real directories instead of trying to hide it all under the
> name of "attributes", IMHO.  :-)

Thanks Curtis,

that makes sense. Forks would be not much different from directories hidden
behind files. From a performance point of view, I think that's just too much
overhead for things like ACLs, which need to be retrieved very quickly.

On ext2, we could try something like the following:

A chained list of blocks (usually just one block) containing something like the
following struct for each attribute:

struct attr_entry {
  __u16    attr_type;     // user or system attribute?
  __u16    name_offs;
  __u32    value_block;   // disk block attribute is stored on
  __u32    value_size;    // size of attribute value
  __u16    value_offs;    // offset in disk block
};

And then somewhere the attribute names:
  "acl\0dacl\0cap\0\0"   // system attribute names
  "thumbnail\0\0"        // user attribute names

Then depending on size, block either points to a direct block, or an indirection
block. With 1K blocks this gives us 256K per attribute. With 4K blocks we could
get 4M. Not bad  :-)

I guess storing the attribute entries consecutively (with the names padded to
word boundaries) should do.

Compared to the current approach, we would have one more level of indirection in
a simple implementation. Maybe that can be improved upon by storing everything
on the first block if there's enough room (which I expect to be the common
case).

Curtis,
is there any much superior approach that I'm not aware of?


Thanks,

Andreas

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 Andreas Gruenbacher, a.gruenbacher@computer.org
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