Re: linux-2.4.10-pre5

Helge Hafting (
Tue, 11 Sep 2001 09:45:48 +0200

Linus Torvalds wrote:

> > An observation: logical readahead can *never* read a block before it knows
> > what the physical mapping is, whereas physical readahead can.
> Sure. But the meta-data is usually on the order of 1% or less of the data,
> which means that you tend to need to read a meta-data block only 1% of the
> time you need to read a real data block.

Seems to me a readahead without metadata don't buy very much. Sure,
you get the file page early without looking up metadata on disk. But
oops - it cannot be used yet as we don't yet know the fact that it _is_
part of the file! When the process gets to ask for that part of the
we still have to wait for metadata.

Physical readahead may or may not help - but I cannot see that this
particular aspect helps anything.

Helge Hafting
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