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mkfs.xfs doesn't detect size of storage correctly

To: xfs@xxxxxxxxxxx
Subject: mkfs.xfs doesn't detect size of storage correctly
From: Matthias Schniedermeyer <ms@xxxxxxx>
Date: Tue, 29 Jan 2008 10:32:01 +0100
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Mutt/1.5.17+20080114 (2008-01-14)

Yesterday i bought a 750GB HDD.
I encrypt nearly everything with loop-aes, so i also did it with this 

I create a "fake" partition table and:
losetup -e aes256 -p 0 -o 4096 /dev/loop6 /dev/sdb < key

This creates a loop with everything except the first 4KB, i.e. it leaves 
out the MBR and another 3,5KB.

/proc/partions shows the correct(tm) size informations for the HDD and 
the loop:
- snip -
   7     6  732574580 loop6
   8    16  732574584 sdb
   8    17  732572001 sdb1
- snip -

But when i mkfs.xfs the loop
#> mkfs.xfs /dev/loop6
meta-data=/dev/loop6             isize=256    agcount=3, agsize=45785911 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=137357733, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096
log      =internal log           bsize=4096   blocks=32768, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=0
realtime =none                   extsz=4096   blocks=0, rtextents=0

And mount it:
mount /dev/loop6 /mnt

And least but not least df it:
#> df -m /mnt
Filesystem           1M-blocks      Used Available Use% Mounted on
/dev/loop6              536426         5    536422   1% /mnt

There is roughly 1/3 missing.

What can i do to fix this?

#> mkfs.xfs -V
mkfs.xfs version 2.9.5
(Package is from: Debian SID/unstable)

Kernel is 2.6.24 (vanilla, plus of course the external module: loop-aes)

Btw. I have 50 other HDDs in more or less exactly the same setup, (49 
HDDs >= 200 GB and <= 500GB and one 1TB, of couse created with my system 
at an older stage from Feb 2007 onwoard, the 750GB is the first with 
kernel 2.6.24.) Up to now i can't remember ever "missing" space.

Bis denn

Real Programmers consider "what you see is what you get" to be just as 
bad a concept in Text Editors as it is in women. No, the Real Programmer
wants a "you asked for it, you got it" text editor -- complicated, 
cryptic, powerful, unforgiving, dangerous.

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