RAID 5/6 Query

rolfharris

rolfharris

rolfharris

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22 Jan 2010
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I'm looking to create a storage server and perish the thought if some of the files were to go missing, so rather than just plonk in a load of drives and away I go I thought I would invest in a RAID setup.

I read through the different types of RAID and understood the simplest where if you had 2 drives and one failed you use the other one.

However what I don't get it RAID 5/6.

If I have 10 identical 1tb drives, and according to the stick the usable disk space would be 8tb, if all the 8tb was used up and two drives failed, how can it be sure to recover all the information lost?
 
OK, basically, if you were to use RAID 6 you will be affecting your performance in a big way, so you need to ues a kick arse controller, enterprise grade.

It really depends how you want to go for RAID, RAID 6 does have advantages, but the lower write speeds, higher system demands and even longer rebuild times, i couldnt imagine how long a RAID 6 rebuild of 5TB would be, let alone 10.

That is another point, you can get 10 port RAID cards, but they are a lot of money, you should be looking at 2 RAID card setups.

If you do some searching you will see a lot of people opt for RAID 6, mainly because they will tell you the worry you have when a drive fails is too great as rebuild times are long on high capacity arrays. RAID 6 lets you have 2 drives fail as opposed to RAID 5's 1.

You have a couple choices really, using an 8 port RAID card have a massive 8 drive RAID 5 setup with a spare drives always on hand for a failure.(6.5TB)
Have 2 RAID 5 arrays on a couple RAID cards.(7.4TB)
Have 1 RAID 6 array of 8 drives (5.5TB)
Have 2 RAID 6 arrays on a couple RAID cards (5.4TB)

The first option has a high risk of failure, i wouldnt use this.
The 2nd option is good, it gives you maximum space but rebuild times are high and it will take more time setting up.
The 3rd is good because it will give you good backup and a good amount of storage, rebuild times will be massive though, risk is higher than the last option.
The 4th option is expencive, but it will give you maximum backup and lower rebuild times than option 3. I realilse giving up almost half of the advertised capacity is hard, but RAID 6 seems to be what you want unless capacity is really an issue, them option 2 will be the best for you.
 
How long would a rebuild take on the 3rd/4th options? It kind of answer my next question which was, when a drive does fail, how does it fix itself.

Also with reference to the 1st reply - I had read the Wikipedia entry but I couldn't work out how it could be sure to recover all data unless you halved your space.

Read/write times are not major point, as long as it can read at rate fast enough to stream HD video recording that's fine.
 
It you have a good system/good controllers, rebuild time for option 3 could be a few days, 80ish hours i think, maybe more, just based on my RAID 5 rebuild times.
For option 4 it will be less, probably 50+ hours. It is the dual parity that adds the time, my RAID 5 with 4 1TB drives is about 36 hours.
 
if one of my drives in my raid 5 array fails (4 sammy F1's), which happens more often than youd think, when my pc crashes (during overclocking or something) and its in use etc. It takes a few hours to rebuild it. There is about 2TB of data on it, and im using the onboard nforce controller. Read speeds are around 270mb/s, sustained write speeds are at around 30-40mb/s using about 10% of my cpu. Found out the write speeds last night while copying 588gb of game installs to back them up :)
 
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