[mythtv-users] INFO: RAID comparison for MythTV

[MagicITX]

On 4/21/05, Allan Stirling <Dibblahmythml0015 at pendor.org> wrote:
> MagicITX wrote:
> > A week or so ago there was a thread discussing RAID for MythTV.  We
> > did a test comparing software RAID 10 and RAID 5 on a mythbackend
> > server. You can find the article on our site if you're interested.
> > Basically we found that the parity calculation for RAID 5 had a
> > minimal impact on the performance of the system.  We did see much
> > better throughput under RAID 10 as one would expect.
> >
> Not bad. However, you say 75% of the storage is available for data. This
> is incorrect.
> 
> I have a 9 drive raid5, with 200Gb drives. One is hotspare, leaving 8
> drives in the array. I have 1.4Tb of storage available.
> 
> As you increase the number of drives, the overhead of raid5 decreases -
> This applies to the CPU overhead for the parity stripe as well as the
> impact on storage available.
> 
> Also, more spindles == more performance, almost always. Going for
> smaller drives will get you more "bang for the buck" than larger, as
> long as you have plenty of controller ports available.
> 
> As another hint - Start your RAID small and grow it. Buying your disks
> all at once means that they'll all tend to fail at once - Which is not a
> good thing. Unfortunately, there's no easy, reliable way I've found to
> extend a RAID without backing up (somewhere really huge :) and restoring.
> 
> Cheers,
> 
> Allan.
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> 

Thanks for the correction, I'll fix the article.  The storage
efficency for me is 75% because I'm using 4 drives.  The actual
calculation is (n-1)/n * 100 = % efficiency.  With 8 drives as you
have it is 87.5%.

While the number of drives in a RAID 5 array is theoretically
unlimited, some recommend no more than 14 drives.  The problem is RAID
5 is hosed if two drives fail.  The more drives you have the more
statistically likely you are to suffer a two drive failure.

I have seen the argument that you shouldn't get all of your drives at
the same time since they are more likely to fail at about the same
time.  As mentioned above that is bad with RAID 5.

Thanks for the feedback.

-- 
Tim
www.magicitx.com