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SubjectRe: multiply files in one (was GNU/Linux stance by Richard Stallman)
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Jon Bright wrote:
> Larry McVoy wrote:
>
> > THINK. You guys aren't THINKING. It is very FRUSTRATING.
> >
> > If you have 200 small files and you have to go get 200 inodes and 200 blocks
> > to read them in, the inodes and the blocks aren't next to each other, that's
> > 400 disk transactions instead of 1. THINK. PLEASE. This is the third time
> > I've gone over this point.

> And if you have 200 small files spread about in a pseudo-tarball of
> 4000 files? Or say your tarballs only cover 200 files at a time,
> you have 200 files spread through 20 pseudo-tarballs? And just how
> frequent an operation is it for something to read 200 small files
> in? And if it's that frequent, then (admittedly you lose on the
> first time), they'll (hopefully) still be cached for the second and
> subsequent occasions?


Just look at it this way: Suppose the parameters of the disk say that
you can readahead 64k for "Free". Say I have a directory with 500
small files. (whatever the limit is, memory, bandwidth,
hitrate-chances: you need to put a limit on the readahead
somewhere....)
In the old situation that readahead would contain 64 files. So opening
a random other file will have a 10% chance of being in the cache, not
requiring another IO.

In the new situation, the 500 files would only occupy say 150k on the
disk. So the readahead hits 40% of the files, giving a 40% hitrate on
the next file.

The extra hitrate is for free (in terms of IO). That's what makes it
attractive.

Roger.

--
** R.E.Wolff@BitWizard.nl ** http://www.BitWizard.nl/ ** +31-15-2137555 **
*-- BitWizard writes Linux device drivers for any device you may have! --*
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