Re: strange problem with raid6 read errors on active non-degraded array

[Roman Mamedov <rm@romanrm.net>]

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On Wed, 02 Jul 2014 10:32:41 +0100
Pedro Teixeira <finas@aeiou.pt> 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

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