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Subject: Re: Linux slack space question
Date: Fri, 8 Dec 2006 13:51:44 -0500
Message-ID: <Pine.LNX.4.61.0612081337510.20280@chaos.analogic.com>
In-Reply-To: <3c698a820612080921u20a957d9x1ac1e01e6734d025@mail.gmail.com>
Thread-Topic: Linux slack space question
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References: <3c698a820612080921u20a957d9x1ac1e01e6734d025@mail.gmail.com>
From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Maria Short" <mgolod@ieee.org>
Cc: <linux-kernel@vger.kernel.org>
Reply-To: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
Sender: linux-kernel-owner@vger.kernel.org
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On Fri, 8 Dec 2006, Maria Short wrote:

> I have a question regarding how the Linux kernel handles slack space.
> I know that the ext3 filesystems typically use 1,2 or 4 KB blocks and
> if a file is not an even multiple of the block size then the last
> allocated block will not be completely filled, the remaining space is
> wasted as slack space.
>

Not wasted, could be extended if additional data are written.

> What I need is the code in the kernel that does that. I have been
> looking at http://lxr.linux.no/source/fs/ext3/inode.c but I could not
> find the specific code for partially filling the last block and
> placing an EOF at the end, leaving the rest to slack space.

An EOF? Unlike CP/M the Linux file-systems copy to user-space up to the
last byte written to the file, not up to the last block. Therefore, there
is no need for "fill" and certainly no EOF character. All Linux/Unix
files are binary files, i.e., there are no special characters inserted.
Now, when you read a file using buffered I/O (the f***() functions), the
'C' runtime library converts I/O so that functions like feof(*stream) work.
The actual EOF on a binary file occurs when a read() returns 0 bytes.

The number of bytes actually written to files are handled in inodes. In fact, 
you can make a file larger simply by moving a file-pointer. That changes
the inode value. Such files are called sparse files and, when read, the space 
not written is cleared so the user never reads something that wasn't
specifically written.

  >
> Please forward the answer to mgolod@ieee.org as soon as possible.
>
> Thank you very much.
> -

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.68 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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