Re: [Yaffs] Extended attribute support

On Tuesday 11 March 2008 14:38:27 Spencer Shimko wrote:
> Hi,
>
> I've started looking into squeezing extended attribute support (xattr) into
> YAFFS. Specifically I would be working on the generic xattr support plus
> support for the user, trusted, and security namespaces. I've already got
> some ideas floating around in my head but I wanted to see if anyone on list
> has explored this before.

I must first declare that I only have limited knowledge of xattr.

I have done some thinking on this and a lot comes down to whether you want to
have fully capable xattr (almost a full database system capable of megabytes
per file) or are prepared to accept a limited xattr system that will probably
work for most cases.

The biggest question is: How big does the xattr blob **need** to be? ie
practical size rather than theoretical.

If xattr can be kept small (which I believe will serve most uses), then it is
almost trivial to store the xattr as a blob in the object header. For
example, if you have 2k nand pages then there's approx 1.5k of space in the
objectheader that does nothing. Is 1.5k enough to store xattr?

The next more complex (but still pretty simple) approach is more powerful.
Currently chunkId offsets start at 1 (chunkId 0 == object header). We can
easily start that at a bigger offset, say 1024, and use chunkIds 1..1023 to
store xattr.

I think the major issue is to try keep things simple. If the fs can just
handle a blob (ie change one xattr means rewriting the blob) then that's a
lot simpler than trying to make file system within a file system to handle
xattr.

The simplicity approach is fine if the xattr are smallish.



>
> I briefly thought I could get away with adding a small blob to the
> yaffs_DeviceStruct to handle the device-level data and adding an xattr
> object variant to handle the raw data. But now I'm not so sure since
> xattr's aren't objects in the "object variant" sense.
>
> 1. Does the relationship between yaffs_Objects (parent, siblings, children)
> preclude implementing xattrs as an object variant?
>
> 2. I have seen the ChunkCache but I don't think the xattr should be
> polluting that cache. Right?
>
> 3. To actually get the chunk data into NAND I could use ObjectHeaders (and
> the associated Object) or raw data. The way the "name" member of the
> ObjectHeader struct is used in other parts of the code
> (yaffs_FindObjectByName) leads me to believe I can't/shouldn't be using
> them for xattr's. Correct?
>
> Aside from those questions, anyone aware of any work going on in this area?
> Any show stoppers jump to mind or suggestions?
>
>
> Thanks,
> --Spencer
>
>
>
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