On Wed, 02 Jul 2014 10:32:41 +0100
Pedro Teixeira [off-list ref] wrote:

- I'm having the following problem on a raid6 md volume consisting og  
16 1TB Seagtes SSHD's. ( using kernel 3.15.3 or 3.14.0 ) mdadm is 3.3.

  - every time I run a fsck.ext4 I will get the exact same errors (  
...short read ). Forcing a repair on the md0 volume shows no errors  
and completes without problems. All disks are active and the volume is  
not degraded, still I can't get rid of the short errors on those 16  
blocks and when the filesystem is mounted the read errors will come up  
from time to time as they are probably in use.

Are you sure that Ext4 in your kernel, and all tools that you use with it (such
as the fsck) really support 16 TB filesystems? I recall there have been some
semi-obvious problems with that. Try a different FS, e.g. XFS or Btrfs instead
of Ext4.

- If I try to read those blocks with DD  ( dd if=/dev/md0  of=test.txt  
seek=458227712 count=6 bs=4096 ) it will instantly create a 1.8T file  
but the file doesn't appear to have nothing on it ( and the file  
doesn't take the 1.8T on disk as the disk is much smaller )

root@nas3:/# dd if=/dev/md0  of=teste.txt seek=458227712 count=6 bs=4096
6+0 records in
6+0 records out
24576 bytes (25 kB) copied, 0.0019239 s, 12.8 MB/s
root@nas3:/# ls -lah teste.txt
-rw-r--r-- 1 root root 1.8T Jul  2 10:22 teste.txt

Here you need to use skip=, not seek=. See "man dd".

-- 
With respect,
Roman