Re: dos filesystem check

[Andrew Clausen]

On Wed, Mar 20, 2002 at 10:40:36AM +0000, Richard Hirst wrote:
> Hi,
>   If you create a file system with an odd number of sectors and then use
> the linux mkdosfs to create a FAT filesystem on it, and then do a parted
> check on that, you get:
> 
> > address@hidden:/build/dosfstools/dosfstools-2.8$ sudo parted -s /dev/sdc
> > check 1
> > Warning: Partition size (64197 sectors) and filesystem size (64196 sectors) 
> > do not match.
> > You found a bug in GNU Parted.  Please email a bug report to address@hidden 
> > containing the version (1.4.24), and the following message:
> > 
> > There are no possible configurations for this FAT type.
> > address@hidden:/build/dosfstools/dosfstools-2.8$ echo $?
> > 0
> 
> mkdosfs actually does a binary search up the disk to find the highest
> numbered sector it can read, and linux doesn't let you read the last odd
> sector of a block device directly.  So, mkdosfs concludes that the
> partition size is one sector less than the real size.  A bug in mkdosfs,
> maybe; parted behaves like Microsoft s/w in this case.

Definitely a bug in mkdosfs... MS s/w might corrupt/misinterpret
the mkdosfs file system.

> Also, having decided there is a bug, parted really shouldn't exit with
> code 0.

Weird... it should!  Could you please debug this?  I can't reproduce
(easily).  If it's too much trouble for you, I can investigate.

> In addition:
> 
> > (parted) p
> > Disk geometry for /dev/sdc: 0.000-4095.856 megabytes
> > Disk label type: msdos
> > Minor    Start       End     Type      Filesystem  Flags
> > 1          0.031     31.376  primary   FAT         
> > 2        101.975    196.105  primary   FAT         
> > 3        298.081    902.087  primary   FAT         
> > 4        996.218   4000.561  primary   FAT         
> > (parted) mkfs 3 fat  
> > Warning: Would you like to use FAT32 for this filesystem?  It is much more 
> > efficient with your disk space, but is not compatible with early versions 
> > of Windows 95 and Windows NT.  Only select yes if you only use Linux, BSD, 
> > MS Windows 98 and/or MS Windows 95 B.
> > Yes No Cancel ? y
> > (parted) check 3
> > Warning: File system is reporting the free space as 154493 clusters, not 
> > 154318 clusters.
> > Ignore Cancel ? 
> 
> So something seems amiss with it's creation/checking of FAT32.  All this
> with parted 1.4.

Right.  I fixed this in 1.6.x, but I want to "make sure" it's right
before backporting.  (I haven't heard any complaints about 1.6.x,
so maybe it's ok?)  Making/resizing FAT file systems is very tricky,
and there is no documentation... (The problem isn't well defined)

This case doesn't arise very much, so it hasn't been a practical
problem.  Very long story.  (see the archives, and comments in
libparted/fs_fat/calc.c)

Maybe I should be more scientific about it... it's a rather long
and tedious job of trying lots of different partition sizes,
and checking how chkdsk & friends react.  The problem is, they're
not very verbose, so there's a bit of math involved too... :(

Andrew