[Gllug] Large raid 5/5+0 setup

[Justin Perreault]

On Fri, 2009-06-19 at 11:19 +0100, James Courtier-Dutton wrote:
> 2009/6/18 Justin Perreault <justinperreault at dl-jp.com>:
> > I've started searching the net to get a better understanding about raid
> > 5/5+0 and the hit I can expect on performance.
> >
> > 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.

That is helpful to know. I had always viewed streaming as a stream of
small bits not as an expanding file.

> 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.

I have started to consider this however once I started looking at
goodish cases I also noticed the higher end gaming cases. This has me
thinking that one high end case that is meant to cool graphics cards
might be sufficient to handle the heat from lots of drives.

I have also considered this from the point of view that having 2+ full
PCs would allow more expansion card slots. As I have not sorted out what
mother board to use I am not sure I will be able to fit all the cards I
need in one machine. This brings up concerns over how seamlessly MythTV
will handle the separate boxes in coordinating the recording of shows.

I'll look into mythtv's multiple master servers handling more this
weekend.

> 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.

Raid 6 seems viable for the number of drives I am planning to use. I
started thinking that raid 5+0 would provide the redundancy similar to
raid 6 with an increase in speed for reading/writing. 

-Justin



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