[openbeos] Re: FS attributes
- From: "Manuel Jesus Petit de Gabriel" <freston@xxxxxxxxxxx>
- To: <openbeos@xxxxxxxxxxxxx>
- Date: Sun, 6 Jan 2002 16:39:11 -0800
Just a couple of corrections:
> I think that there is a little more to it than this.
> First off, some amount of attributes are stored in the directory structure
> (the inode). 700 bytes or so. So many times, the attributes cost you *no*
It's around 700 bytes for file systems initialized to 1K blocks. For 2k
block
size file systems it is 1024+700, for 4k blocks....
> file system space. That space is used, regardless. Secondly, if there are
> more than 700 or so bytes, you would have to read in another block
It's worse than that. If there are attributes that cannot be embedded into
the inode an attribute directory is allocated, so you BFS internally will
have to fetch the inode for the attribute directory, and later fetch the
directory
data for walking it (the attribute directory looks structurally like a
regular
directory). Now for each attribute you are walking the inode of the
attribute will have to be fetched to find out the size. Note that those
attributes look structurally like a regular file.
> to find out how big the file is. That would *dramatically* increase the
> time it takes to read a directory. Third, while attributes *CAN* be used
> to hold mountains of data, it is, IMHO, a bad idea. They aren't designed
to
> do so well.
That's right. While the structure allows for unbounded attribute
prolifferation,
the performance and space usage will suffer badly. Note that each attribute
not embedded will take on its own (not counting the storage requirements
for the attribute directory) 2 blocks: 1 for the attribute inode and 1 for
the attribute data itself. An optimization that was in the TODO list was
to embedd the atribute data in the attribute inode for small enough
attributes.
Summing it up: while having huge attributes is ok (performance and storage
wise),
having tons of small attributes is bad (storage wise, and performance wise
if you
want to scan all attributes).
manuel,
>
>
>
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