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Ubuntu Desktop RAID5 Best Practices

Posted on 2011-03-09
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Last Modified: 2012-05-11
Hey there Experts,

I'm getting ready to build an Ubuntu Desktop box that I'll be using for a virtual machine server using Ubuntu desktop and virtualbox.  I would like to create a RAID array to allow for hard drive fault tolerance but I'm a little shaky on the details of this, especially in Ubuntu.  

I have 3 500GB SATA drives that I would like to use for the array.  I also have a 250GB SATA drive which came with my server.  I think I would like RAID5.

I guess the most basic question is this:
Should I add the 250GB drive, install Ubuntu on it, and then create my array from the three 500GB drives to hold my VMs?  or

Should I just leave the 250GB drive out of the picture and install Ubuntu and my array on the three 500GB drives.  

It seems counter-intuitive to me to use the 250GB for the OS but I have seen this setup referenced on the web.  

Anyway, any help you can give will be greatly appreciated.  

Thanks!

PS - there is no hardware RAID controller and the budget is shoestring so we'll be sticking w/ software RAID.  
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Question by:ttist25
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9 Comments
 
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by:edster9999
edster9999 earned 1000 total points
ID: 35082917
hardwar raid is better - for speed and for data safety.  But if you are under budget then the software version does almost as good job.
The one situation that could come up would be for the software to have a bug and to write across all the drives corrupting your Raid Array and making it worthless.
The hardware one could have bugs in its firmware but would be less open to software going wrong and doing the damage.
But - I think that has already been decided so I'll move on.

Raid 5 will take away the storage space of one of the drives.  So your 3 x 500 GB will give you 1000 GB (1TB) of storage.

As the raid storage was more expensive (and slightly slower) this was seen as waste if it was doing silly things like keeping the OS swap or even the OS files etc.
A standard setup would be to have a boot disk with the basic OS on it and the data on a raid array.  people sometimes did a simple mirror (raid 1) on 2 disks and made these the OS set and then 3 or more disks for the data as Raid 5.

With faster machines doing software raid  / hardware raid and the falling price of hard disks machines are often just setup now with 1 x big Raid array with all of the above inside it.
This does cause a slightly harder debug if the array goes corrupt or need rebuilding as the OS is inside it but can normally be done.

...and... don't foegt.  You still need backups.  1 - The raid can fail and stripe all your disks with bad data.  2 - You can delete or corrupt files.  the raid array does not let you roll backwards, a tape does.

Have fun :)

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Expert Comment

by:Martin_J_Parker
ID: 35083067
Yes, I would leave the 250GB drive out of the equation otherwise your opsys won't be protected.

Does your motherboard support RAID 1 (mirroring) or RAID 10 (mirror + stripe)?
If it does a 4th 500GB disk might be a worthwhile investment as a RAID 10 over 4 disks would give you a far faster disk access than RAID 5 as there is no parity bit to calculate - although it would, of course only give you 1TB of storage from the 4x 500GB disks.

Considering that an additional drive is only about £30 including VAT you might be able to afford that: http://www.ebuyer.com/product/241715
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by:ttist25
ID: 35083314
Thanks guys.  I'm pretty much penned in budget wise so it will have to be software with the disks I have.  

Given what I understand so far you're both recommending I keep the OS on the three 500GB's in a RAID array (1TB is plenty of space so I'm assuming RAID5 is the way to go).

Is that right?  

I've asked another question re: when to setup the array and that's available here if you want to answer.

Thanks again!
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Martin_J_Parker earned 1000 total points
ID: 35083616
Yes, that's right.

When you say 1TB is plenty, does that mean you've got loads of space to spare?

Does your motherboard and BIOS support RAID 1 (mirror)?

If the answer to both questions is "yes" then another option might be to use the 250GB disk with one of the 500 MB drives to give a 250GB mirror for the opsys, swap and temp areas and the other 2 drives could be mirrored for a 500GB data area.
 
That would leave you with quite a bit less available disk, but it would run quite a bit faster, especially as the CPU wouldn't be doing any of the RAID work , and it wouldn't be as hard to debug if you got problems.

Some RAID controllers do insist on having 2 drives the same size for mirror, but some do allow a mirror of the smaller sized drive.
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Author Closing Comment

by:ttist25
ID: 35094692
Thanks guys - I'll post a new question as you guys have gone above and beyond on this question and I have found that I actually DO have an S100 PERC controller integrated into the MOBO.  Initially Dell rep said no, but after chatting with a different rep, it turns out that there is an integrated controller.
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Expert Comment

by:Martin_J_Parker
ID: 35094756
Great.  According to Dell the Perc S100 does support RAID 5 so that's just what you need:
http://support.dell.com/support/edocs/software/svradmin/6.2/en/OMSS/apndx.htm#wp1089749
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by:ttist25
ID: 35095522
I think my only issue there is can I get it to work with Ubunutu
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Expert Comment

by:Martin_J_Parker
ID: 35095655
Hmmm - looks like it only works with Windows from what I can see on the net.
That's very annoying - unless you use Windows and run Linux in a Virtual Machine - which seems like an awkward way of doing things!
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Author Comment

by:ttist25
ID: 35099616
Yeah - I just setup RAID5 with 3 disks.  Seems to be working.  I just shutdown and cut power to one of the drives and it dropped to initramfs.  

I guess this is where I could rebuild it?  That's another question.

Thanks again for everything.
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