[Gllug] Large raid 5/5+0 setup

[James Courtier-Dutton]

2009/6/18 Justin Perreault <justinperreault at dl-jp.com>:
> Hi everyone,
>
> I've started searching the net to get a better understanding about raid
> 5/5+0 and the hit I can expect on performance.
>
> I might be barking up the wrong tree and if so please point me in a more
> useful direction.
>
> The purpose of my searching is to sort out my myth-tv needs. I am
> currently thinking that I would like to have 20 x 1TB SATA drives in a
> raid 5+0 configuration for my backend server which would be responsible
> for the recording of all of the shows/channels I currently track and
> distribution to the front end machines where I would be able to watch
> what was recorded. (I might consider 1.5TB drives but I am already
> getting concerned about a system being able to handle it all)
>
> There would be an average of about 3250 Kb/sec written to the storage
> with peaks of 5800 Kb/sec.
>

I would take the simple approach and just purchase a few PCs and just
put 5 HDs in each for RAID 5. Make sure the PC case is a goodish one
with lots of air flow due to heat produced by those HDs.
Your application is streaming, so using RAID 5 will not slow it down
at all really.
RAID 5 slows down systems that make small read/write transactions
because the RAID has to read the entire stripe for each transaction.
That gives you 4 PCs with 5 HDs each. Once one PC get full, you just
move onto storing the programs on the next. This would even let you
stage the purchases because 4TB will take a while to fill. Get HDs
that have 5 year warranty, that should give you a good MTBF.
There are other RAID options that provide better redundancy as the
number of disks increase.
One problem you might start getting is bit errors. A HD bit error rate
might be starting to get to the point where a bit error is almost
guaranteed during a HD rebuild. This could push you to have to use
RAID 6. I.e. 2 checksum disks instead of the 1 with RAID 5.
So, with RAID 6, you would have 4 PCs with 6 HDs in each.

Kind Regards

James
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