VFAT: slow fs corruption? [long]

["Albrecht Dreß" <albrecht.dress@arcor.de>]

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Hi all,

first, please excuse me if this is a very dump question...

I use Linux (inter alia) on an ARM9 system which is attached to a  
measurement device.  The device produces a new data set of ~10 kByte about  
every 20 seconds, and the ARM system stores the data on a 1GB SD card  
attached to it.  The kernel is unfortunately rather old - 2.6.11,  
including several vendor (Digi) supplied drivers.  I didn't have time yet  
to port them all to a recent kernel.

My first approach was to use the SD card with a VFAT file system on it, as  
such cards usually come pre-formatted with it and as the VFAT cluster size  
of 16k matches the erase block size of the card, which should help its  
built-in wear leveller to do its job.

After some time, though, I noticed that several files with measurement  
data were empty, although the directory indicated that they were not.  The  
system was not switched off, and I always call sync() after producing a  
new file.  Running dosfsck on the file system, I got several errors like

   File size is 13978 bytes, cluster chain length is 0 bytes.
   Truncating file to 0 bytes.

or

Cluster 0 out of range (4849729 > 247450). Setting to EOF.
Orphaned long file name part "5d"

My first idea was that the (Digi supplied) MMC driver is somewhat broken.   
Just to check that, I re-formatted the SD card with ext2, and ran the test  
again - and all files looked fine!  Called "e2fsck -f" - no problems with  
the file system.  That seems to indicate that the MMC layer /does/  
actually work.

Questions:
- Is the conclusion correct that the problem seems to be in the VFAT part,  
not in the MMC layer?
- Are there known issues with VFAT in 2.6.11 which might lead to the  
observed problems?  Were they fixed?
- Is it possible to change the block size in ext2 to 16k (to match the SD  
card's erase block size)?

Thanks in advance for any help,
Cheers,
Albrecht.


--
  Albrecht Dreß  -  Johanna-Kirchner-Straße 13  -  D-53123 Bonn (Germany)
        Phone (+49) 228 6199571  -  mailto:albrecht.dress@arcor.de
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