Re: filesystem corruption with md raid6

[Bill Davidsen <davidsen@tmr.com>]

Neil Brown wrote:
> On Thursday April 26, pryke@focus.uchicago.edu wrote:
>   
>>  have a system with 12 SATA disks attached via SAS. When copying into the 
>> array during re-sync I get filesystem errors and corruption for raid6 but not 
>> for raid5. This problem is repeatable. I actually have 2 separate 12 disk 
>> arrays and get the same behavior on both.
>>
>> Does this sound familiar to anyone?
>>     
>
> Yes.
>
>   
>> Here's a little more detail:
>>
>> - 8 core AMD64 system running RHEL4U4 kernel 2.6.9-42.0.3.ELsmp
>>     
>                                                ^^^^^
>
> 2.6.9 is very old.  If you want support for RHEL, you should really be
> asking RedHat.
>
> Tell them it might be fixed by 
>    http://linux.bkbits.net:8080/linux-2.6/?PAGE=gnupatch&REV=1.1938.340.65
>
> (I'm not sure how stable that URL is ... maybe take a copy now. it
> should start
> #### ChangeSet ####
> 2004-11-11 13:48:33-08:00, neilb@cse.unsw.edu.au 
>   [PATCH] md: fix raid6 problem
>   
>   Sometimes it didn't read all (working) drives before a parity calculation.
> )
>
> but I cannot be certain (it was a long time ago).
> That patch came out after 2.6.9 but about 2 months.
> So another option is to build a more recent kernel.
>   

Generally if you run RHEL you don't build a newer kernel. Both because 
there are often stability issues being addresses by using RHEL, and 
because RH kernels are often patched in odd ways compared to mainline 
and the system may expect subtly different behavior from the kernel.

For what RH charges, send them a copy of Neil's possible issue and let 
them fix it.

Oh, and dropping a lot of data into a rebuilding array is generally not 
desirable, often the total time will exceed rebuild and update as serial 
operations. When it comes to write performance RAID6 is good for 
reliability. ;-)

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979