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Re: Idea for reducing disk IO on tagging operations

From: Spiro Trikaliotis
Subject: Re: Idea for reducing disk IO on tagging operations
Date: Wed, 30 Mar 2005 14:43:22 +0200
User-agent: Mutt/1.5.6+20040907i


sorry for the late reply to this, but I was on vacation. Anything, I
believe I might be able to contribute something to this discussion,
which even resulted in some code.

* On Sun, Mar 20, 2005 at 11:54:32PM +0000 Dr. David Alan Gilbert wrote:

> OK, my conscience will let me carefully ignore NFS issues given the
> pain it causes me elsewhere (and I make my mechanism switchable).
> What happens if I only used the overwrite mechanism if
> none of the characters being modified crossed a 512 (e.g.) byte
> boundary offset in the file?  Since the spaces were actually
> written in a previous operation we can assume that the space
> is allocated and no allocation operation is going to happen
> at this point (mumble filesystem journalling mumble!).

IMHO, here, you are not correct. If I write X times a char Y into a
file, I cannot assume that memory for X characters has been allocated.
The file system can do some optimizations, compress the file (for
example, run-length encoding RLE: First character tells that X times the
same character will be written, and the character itself is written
afterwards), or anything else. Furthermore, think of so-called
"sparse-files", which can be rather big - much bigger than your actual
medium is itself.

Because of this, even a block boundary in the file does not make much
sense, IMHO, for the general case, that is, arbitrary file systems.


Spiro R. Trikaliotis                               

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