[mythtv-users] Hard Drive reliability esp. RAID issues

Stef Coene stef.coene at docum.org
Mon Mar 8 17:15:55 UTC 2010


On Monday 08 March 2010, John Drescher wrote:
> > I am sure you will all have seen issues with RAID and dive failure;
> > with the readership of this list. I have just seen this article
> > covering the RAID5 issues and RAID6 lifespan.
> > 
> > The highlights are:
> > 
> > Today; RAID 5 with moderate amount of drives is likely to fail during
> > a rebuild following a drive failure; this has been well publicised
> > because of the error rate of drives.
> 
> I would say from experience that this is very unlikely that a second
> drive will fail in the 8 hours it takes to rebuild a 6TB array. And
> even if it did with linux software raid you could recover. With HW
> raid you will probably need specialized software to force the array to
> except drives that are marked bad.
The problem is not that a second drive will fail.  The problem is, statistical 
speaking, that 1 bit of 10 TB is unreadable, the so called UER (uncorrectable 
error rate).
See this document from 2005 http://arxiv.org/pdf/cs/0701166

This means that if you have to rebuild a 10 TB raid, you have to read 10 TB to 
calculate the parity bit.  But 1 bit will not be readable, so bye bye raid 5.

What if you have a 10 TB disk?  Then a simple mirror will not be enough 
because you can never read the disk without missing 1 bit.

The UER is getting better and better, but it is like the speed of a disk.  The 
speed and the UER are not catching up with the size of disk.

So yes, raid 6 will be good for the next future.  But once there are 10 TB 
disks, even a mirror will not be enough.


Stef


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