Re: [PATCH-BK-2.6] NTFS fix "du" and "stat" output (NTFS 2.1.6).

[Anton Altaparmakov]

On Wed, 2004-01-21 at 18:24, David Sanders wrote:
> On Monday 19 January 2004 09:15 am, Anton Altaparmakov wrote:
> > This fixes the erroneous "du" and "stat" output people reported on
> > ntfs partitions containing compressed directories.
> 
> Thanks for the quick patch.  There are still problems with the reported 
> disk usage.

No there are not.  It is a feature, not a bug.  (-:  I will explain
below...

> I use as an example the file win.ini.  With the 2.4.24 
> kernel I get the following results:
> $ ls -l win.ini
> -r--r-----    1 root     staff         399 Jan 27  2003 win.ini
> 
> $ stat win.ini
>   File: "win.ini"
>   Size: 399       	Blocks: 2          IO Block: 1024   Regular File
> Device: 305h/773d	Inode: 1023        Links: 1
> Access: (0440/-r--r-----)  Uid: (    0/    root)   Gid: (   50/   staff)
> Access: Thu Jan 15 15:34:09 2004
> Modify: Mon Jan 27 18:54:00 2003
> Change: Sun Sep 22 07:23:44 2002
> 
> $ du -h win.ini
> 1.0k	win.ini

This is wrong.  The file size is 399 bytes,  and knowing from my own
systems, win.ini is a resident file (which is consistent with the small
size).  This means the file doesn't occupy any disk blocks at all.  All
the data is stored inside the on-disk inode itself (that is the MFT
record in NTFS speak).

Further, the table of inodes (the system file $MFT) already has its own
size (do "ls -l \$MFT" or "du \$MFT" in the main NTFS directory and you
will see it together with its size). 

The size of the inode itself (1024 bytes) is already accounted for in
the size of the $MFT system file (the table of inodes).

Since the 399 bytes of win.ini are part of those 1024 bytes we would be
counting them twice if we were to give them in the "Blocks:" field of
stat, which is what "du" uses to calculate sizes.

So by returning the above "Blocks: 2", the old NTFS driver (which is
what you used by using an unpatched 2.4.24 kernel), du will actually
tell you that you are using more space than you are as it would count 2
Blocks from win.ini and then the same 2 Blocks from $MFT which clearly
is silly.  (-:

> But, under the 2.6.1 kernel:
> $ ls -l win.ini
> -r-xr-x---    1 root     staff         399 Jan 27  2003 win.ini
> 
> $ stat win.ini
>   File: "win.ini"
>   Size: 399       	Blocks: 0          IO Block: 4096   Regular File
> Device: 305h/773d	Inode: 1023        Links: 1
> Access: (0550/-r-xr-x---)  Uid: (    0/    root)   Gid: (   50/   staff)
> Access: Thu Jan 15 15:34:09 2004
> Modify: Mon Jan 27 18:54:00 2003
> Change: Mon Jan 27 18:54:00 2003

Correct.  No disk blocks are occupied as explained above.

> $ du -h win.ini
> 0	win.ini

That is correct.  To quote from the man page for "du":

	"Summarize disk usage of each FILE"

It is "disk usage" not "file size" and the file "win.ini" does not use
any disk space as explained above.

> Now, surely the 2.4.24 kernel is reporting the more accurate disk usage 
> since with 2.6.1 it reports 0 blocks (vice 2).

Nope.  Sometimes things are not what they seem.  (-:

Hope this clears the confusion up.

Best regards,

	Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ &
http://www-stu.christs.cam.ac.uk/~aia21/


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