Re: Bug in block device read/write!

From: tytso@mit.edu
Date: Tue Sep 12 2000 - 23:01:50 EST


   Date: Fri, 08 Sep 2000 03:41:27 +0100
   From: Anton Altaparmakov <aia21@cam.ac.uk>

   I have been trying to get the linear md driver to work with NTFS volumes
   for several months and it never worked. - I was suspecting the NTFS driver
   (after having fixed linear md and verified that at least that worked fine)
   but today I finally found why it doesn't work:

   There is a bug in reading/writing to block devices. - It manifests itself
   in the form that partitions are too small by exactly one sector!

   Even though a cfdisk shows that a partition has a certain number of
   sectors, you can never seek + read and/or write to the last sector (doing
   file i/o using read/write(2) [also tried fread/fwrite(3), same result]. -
   Last sector doesn't seem to exist. However reading the actual hd (/dev/hdb
   or /dev/sda, ie. affects both IDE and SCSI) instead of the partition
   (/dev/hdb7 or whatever) the sector does exist and contains the expected
   information!

This isn't a bug. The last sector is used by the md device to store the
md superblock.

                                                - Ted
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