Linux slack space question

Linux slack space question

[Maria Short]

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.

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.

Please forward the answer to mgolod@ieee.org as soon as possible.

Thank you very much.

Re: Linux slack space question

[Bob Copeland]

On 12/8/06, Maria Short <mgolod@ieee.org> wrote:
> 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.

There is no place where it writes an EOF.  The size of the file is
stored in metadata (e.g. inode->i_size), and only the appropriate
number of blocks up to i_size are read or written to.  Look at
ext3_get_block to see how blocks are read and allocated.

Bob

Re: Linux slack space question

[linux-os (Dick Johnson)]

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/
_
\x1a\x04

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Re: Linux slack space question

[Erik Mouw]

On Fri, Dec 08, 2006 at 12:21:04PM -0500, 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.
> 
> 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.

Think about it: what value would an EOF have if all byte values are
allowed in a file?

>From the very first Unix filesystem an inode contains both the number
of blocks it contains and the actual file size.

> Please forward the answer to mgolod@ieee.org as soon as possible.

Hmm no. You asked a public forum so the reply will go to that same
public forum. See http://catb.org/esr/faqs/smart-questions.html#noprivate .


Erik

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