Author Topic: mixing pru and dru across 16 drives.  (Read 4786 times)

Offline stevewp

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mixing pru and dru across 16 drives.
« on: October 22, 2011, 10:56:59 pm »
I am considering creating an array of 16 1TB drives partitioned 75%Dru and 25%Pru per disk.
partitioned this way can you see any problems with the installation ?
I would think that spreading around the parity across all volumes would be much safer. No I really don't care all that much about speed.

Offline someone

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Re: mixing pru and dru across 16 drives.
« Reply #1 on: October 23, 2011, 12:23:20 am »
No, you just have to figure out how you gonna put those 32partitions into how many DRUs and PPUs.  Aside from performance, you probably can't assign them into drive letters, but mount on to folders instead.  Then if you need to use utility software it may just crash as it can't assign that many drive letters.  And unless you already own those 16x 1TB drives, it's probably be cheaper(cost per GB) getting 2TB drives instead.  And no, this isn't like a real RAID where all your drives have to be the same size.  You can mix and match; you probably already know that but just in case.

Offline Brahim

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Re: mixing pru and dru across 16 drives.
« Reply #2 on: October 23, 2011, 01:02:25 am »
I am considering creating an array of 16 1TB drives partitioned 75%Dru and 25%Pru per disk.
partitioned this way can you see any problems with the installation ?
I would think that spreading around the parity across all volumes would be much safer. No I really don't care all that much about speed.

Yes, there is a huge problem. Losing a drive in such a setup would cause you to lose both data and parity information.

Offline someone

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Re: mixing pru and dru across 16 drives.
« Reply #3 on: October 23, 2011, 02:56:56 am »
But wouldn't you only partially lose some data and some parity any way and not all?
If there are 16 drives, doesn't that mean losing 1 drive mean losing 6.25% of the data and 6.25% of the parity?  Wouldn't you be able to recover if you got 93.75% of the parity to recover 6.25% of the data lose?  Am I missing something?  Can you explain how FR work when you have partial lost?

UPDATE 1:
Oh, I think I missed that this is Real-Time RAID, it work differently.  Thus you can't have that much PPU to recover the break down.
« Last Edit: October 23, 2011, 03:13:22 am by someone »

Offline stevewp

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Re: mixing pru and dru across 16 drives.
« Reply #4 on: October 23, 2011, 06:03:55 am »
Thank you everyone for your input.

 
Yes, there is a huge problem. Losing a drive in such a setup would cause you to lose both data and parity information.

Please explain more about the problems with this setup, I would think that in a worst case scenario of loosing 4-5 drives out of the set I could still theoretically recover the array.

But wouldn't you only partially lose some data and some parity any way and not all?
If there are 16 drives, doesn't that mean losing 1 drive mean losing 6.25% of the data and 6.25% of the parity?  Wouldn't you be able to recover if you got 93.75% of the parity to recover 6.25% of the data lose?  Am I missing something?  Can you explain how FR work when you have partial lost?

UPDATE 1:
Oh, I think I missed that this is Real-Time RAID, it work differently.  Thus you can't have that much PPU to recover the break down.

I have been reading all the information I can scrounge about flex raid, I really wish there was some in depth documentation , this is the first I have heard of limits on ppu useable on recovery, where can I learn more ? Yes I have been to the wiki and wikia and have read it all.  Is there more documentation with the downloaded program ?

Offline Brahim

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Re: mixing pru and dru across 16 drives.
« Reply #5 on: October 23, 2011, 09:16:35 am »
It is not about the amount of data lost, it is about the number of failures.

Losing a drive containing both data and parity is a double failure and would need a RAID engine supporting more than one failure.
Now, if you selected a T2+ engine, which requires 2 parity drives, how could you lay out those two PPUs?

Offline NLS

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Re: mixing pru and dru across 16 drives.
« Reply #6 on: October 23, 2011, 05:08:39 pm »
He probably wants to trick the system into some kind of RAID5 using a RAID4 engine. :)
Interesting thought but no go.
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Offline someone

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Re: mixing pru and dru across 16 drives.
« Reply #7 on: October 23, 2011, 05:42:58 pm »
It is not about the amount of data lost, it is about the number of failures.

Losing a drive containing both data and parity is a double failure and would need a RAID engine supporting more than one failure.
Now, if you selected a T2+ engine, which requires 2 parity drives, how could you lay out those two PPUs?
w/ that many drive, if you have 16DRU each are 750GB, then you could have 5 PPU also each w/ 3x250GB in Tx engine?  No?

Offline stevewp

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Re: mixing pru and dru across 16 drives.
« Reply #8 on: October 23, 2011, 10:16:53 pm »
I think It might be helpful if I explain my situation and then ask for suggestions on setup.

I am a truck driver and am setting up a computer for use as a file-server in the truck.
Semi's are strait axel trucks and what this means is that there is a lot of vibration and vary hard lurching when you hit a bump  or pothole.
Not a great enviroment for hard disks.

My server boot drive will be a small SSD prolly 60 gig or so.
I have 2 jbod sas raid cards 8 port each.

for the actual drives I will be using laptop 1 gig drives, these seem to function pretty well for me in my current laptop in the truck.
2 years now with no failures. I can't afford to stick with all ssd drives I just don't have that kind of money.
1 gig laptop drives are around 100.00 a pop so are cheap enough to replace easily when failures occurs.

I have a huge dvd collection I plan on ripping and putting on the server as well as all my other media, loosing this collection would be for me vary traumatic and expensive to replace.
My desire is for the safest raid setup I can concoct ,  So what would you suggest ?
thank you sincerely for your consideration and advice.

Offline someone

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Re: mixing pru and dru across 16 drives.
« Reply #9 on: October 23, 2011, 10:33:32 pm »
Well, do you very need to take those collection w/ you all the time?
Wouldn't you be better off using standard 3.5" harddrive and setup the server at home, then only take small collection w/ you on each trip?  Maybe 2 or 3  2.5" harddrive as a small server or just a laptop that can take 2 harddrive.  Then you would always have all your collection safer at home.

Offline stevewp

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Re: mixing pru and dru across 16 drives.
« Reply #10 on: October 23, 2011, 10:52:43 pm »
Well, do you very need to take those collection w/ you all the time?
Wouldn't you be better off using standard 3.5" harddrive and setup the server at home, then only take small collection w/ you on each trip?  Maybe 2 or 3  2.5" harddrive as a small server or just a laptop that can take 2 harddrive.  Then you would always have all your collection safer at home.

I dont own or rent a home, I get out of the truck mebby 2-3 times a year for about a week at a time to visit with my mother.
I really do need to take / keep it all with me.

One of my first experiments with a nas in the truck was with a 2, 3.5 drive device, the drives didnt last for more then about 3 months.

Offline someone

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Re: mixing pru and dru across 16 drives.
« Reply #11 on: October 23, 2011, 11:17:18 pm »
3.5" hard disks are designed for desktop/workstation/server which don't require a lot of movement.  Those drives don't have a lot of tolerance against vibration.
The 2.5" disks are designed for notebook, so it's far more tolerance.  Even if the server isn't on while vehicle is moving, the 3.5" drives are still gonna die pretty quick.  So, now I see why you need to use 1TB drives.  But given your scenario, I don't see the need for real time RAID anyway and given the fact that the server gonna be in mobile a lot.  I would make sure I have at least 2 PPU.  Have snapshot raid for your movie/music collection and everything else have duplicate backup on a daily basis aside from the snapshot.  Meaning everything that's in the snapshot raid are write-once stuff.  And I wouldn't leave the server on while the vehicle is in motion, it's a quick way to kill more than 1 drive at a time.  I would follow Brahim advise, setup Tx engine and have like 3 PPU since your chance of data lost are much higher than other people.  But keep in mind that even if HDD don't die, that doesn't mean you can't lose data.  Corrupted partition or bad sector can still render your data inaccessible which could as a fail DRU also.

Offline m0t0k0

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Re: mixing pru and dru across 16 drives.
« Reply #12 on: December 01, 2011, 02:21:14 am »

Have you considered putting some kind of vibration dampening under your server?

Also I can see why you may want to run your server while in transit to record shows or listen to music. You could build a small separate system with a couple of SSDs to act as your playback device and recording machine and then swap the data around after you have stopped.
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