Re: [PATCH] LogFS take three

Jörn Engel
2007-05-16T13:24:57Z

On Wed, 16 May 2007 15:36:44 +0300, Pekka Enberg wrote:
> On 5/16/07, Jörn Engel <joern@lazybastard.org> wrote:
> >
> >More trouble?
>
> Forgot to add (see below). Seems logfs_segment_read would be simpler
> too if you fixed this.

Would it? I think that code would still be needed, although possibly in
a different function.

There are two minor drawbacks to using the page cache, btw:
- Indirect blocks need some mapping too. So either I need to steal a
bit from the inode space or from the fpos space.
- OOM handling is a bit more complicated. I would need a mempool for
that.

> >[ Objects are the units that get compressed. Segments can contain both
> >compressed and uncompressed objects. ]
> >
> >It is a trade-off. Each object has a 24 Byte header plus X Bytes of
> >data. Whether the data is compressed or not is indicated in the header.
>
> Was my point really. Why do segments contain both compressed and
> uncompressed objects?

Compressing random data will actually enlarge it. If that happens I
simply store the verbatim uncompressed data instead and mark it as such.

There is also demand for a user-controlled bit in the inode to disable
compression completely. All those .jpg, .mpg, .mp3, etc. just waste
time by trying and failing to compress them.

Jörn

--
Write programs that do one thing and do it well. Write programs to work
together. Write programs to handle text streams, because that is a
universal interface.
-- Doug MacIlroy