FlexRaid - What Size Does The Parity Drive Need To Be? - AVS Forum
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post #1 of 15 Old 08-06-2010, 03:39 PM - Thread Starter
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I've spent the last couple of hours trying to find a definitive answer to this and still not clear.

I have 3 2TB HDD's, the largest single file on any of the drives is circa 15GB. So what size parity drive do I require as a minimum to provide redundancy against 1 HDD failure?

I have a couple of smaller HDD's lying around which I could potentially use for a parity drive...
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post #2 of 15 Old 08-06-2010, 04:21 PM
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I've been looking into Flexraid for a while, and I've always been told your largest drive must be your parity drive.
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post #3 of 15 Old 08-06-2010, 04:22 PM
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It has to be the same size as your largest disk.
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post #4 of 15 Old 08-06-2010, 04:38 PM
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Parity has to be equal to or greater in size than the largest data drive. You can have multiple parity drives, though I think it is recommended to have one large parity drive.
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post #5 of 15 Old 08-06-2010, 05:36 PM - Thread Starter
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Thanks for confirming that guys. So definitely no chance of being able to use one of my old 1.5TB drives then despite my new 2TB drives only being 50% full at the moment? Time for another 2TB if that's the case (rather not use multiple drives for parity).
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post #6 of 15 Old 08-06-2010, 06:38 PM
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Yes you can use your 1.5TB if your only at 50% on the 2TB.

You will only need a 2Tb HDD for Parity if you go over 1360GB or whatever your 1.5TB HDD is on your 2TB HDDs. Once you use about 70-75% of your 2TB then you will need to go get a 2TB for Parity.
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post #7 of 15 Old 08-06-2010, 08:30 PM
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Quote:
Originally Posted by BDestroyer8418 View Post

Yes you can use your 1.5TB if your only at 50% on the 2TB.

You will only need a 2Tb HDD for Parity if you go over 1360GB or whatever your 1.5TB HDD is on your 2TB HDDs. Once you use about 70-75% of your 2TB then you will need to go get a 2TB for Parity.

+1

The parity data is only as large as the max amount of used space on any single drive.

But it would be easier to go ahead and use a 2TB drive now as the parity and install the 1.5 TB drive as one of the data drives.
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post #8 of 15 Old 08-07-2010, 06:18 AM
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Quote:
Originally Posted by jrwalte View Post

+1

The parity data is only as large as the max amount of used space on any single drive.

But it would be easier to go ahead and use a 2TB drive now as the parity and install the 1.5 TB drive as one of the data drives.

Yup, that way you can just add a new data drive instead of having to copy over the parity drive.
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post #9 of 15 Old 08-07-2010, 06:59 AM - Thread Starter
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Thanks for the replies chaps! Much appreciated. A new 2 TB drive it is then .
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post #10 of 15 Old 08-07-2010, 08:38 AM
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Not to hijack this thread but I was just getting ready to post a similar question. I am still confused on how the parity works for flexraid. For example. I have 3 x 2tb drives containing data and a fourth which is designated as my parity drive. If there is 4tb (3.8 actually) of combined data between the three 2tb drives, how is a single 2tb drive going to have enough capacity to handle parity of the three 2tb drives?

This is where I am getting confused. How can 4tb of data fit on a single 2tb parity drive? I understand parity is not necessarily the sum of the total data but it just doesnt make sense to me.

This is the only thing holding me back right now and I have been researching trying to find the answer.

Thanks guys
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post #11 of 15 Old 08-07-2010, 09:18 AM
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In simple terms, the parity looks at the same bit block on each HD and sees if it is a 1 or a 0. It sums this total and stores it on the parity drive. If one of the drives fail, the parity will look at the other HDs bit total and compare it to the parity. If it matches, then that bit on the missing drive was 0. If it's short one, it knows the bit was 1.

FlexRaid's parity works well. I've restored 930GB of data on a HD.
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post #12 of 15 Old 08-07-2010, 11:49 AM
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Okay thanks. So in my example this is what I have found.

I have 3.8tb of data across three 2tb data drives and than a dedicated parity drive of 2tb. I copied all my movies (3.8tb) worth none larger than 6gb (have excluded the hd for now). My parity drive has filled to only have 127gb free.

So I guess my question is, is this normal? If a 2tb drive should be able to handle 3 x 2tb drives with full data, I am just about maxed on my parity drive and still have another 1.5tb of space on my data drives.

This is where some of the confusion is coming in.
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post #13 of 15 Old 08-07-2010, 12:00 PM
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First, I'd suggest you do some reading on RAID, FlexRAID works exactly like normal RAID in this regard.
Quote:
Originally Posted by a_user View Post

So I guess my question is, is this normal? If a 2tb drive should be able to handle 3 x 2tb drives with full data, I am just about maxed on my parity drive and still have another 1.5tb of space on my data drives.

This is where some of the confusion is coming in.

As jrwalte said, RAID works by summing the contents of the other drives, and writing the sum to the parity drive.

A simple breakdown of how parity works across 4 drives is:
Code:
A   B   C   Parity
0 + 0 + 0 = 0
1 + 0 + 0 = 1
0 + 1 + 0 = 1
0 + 0 + 1 = 1
0 + 1 + 1 = 0
1 + 1 + 0 = 0
1 + 0 + 1 = 0
1 + 1 + 1 = 1
If one of your drives dies the RAID software (or hardware) can take the contents of the Parity file on the last drive, subtract the contents of all the other existing drives, and calculate the value that should have been present on the missing drive. Empty space on one of the drives is simply a whole lot of zeros as far as RAID is concerned, so the parity drive will fill up with a file to get the sum total for all the other drives combined. That sum total includes the 0's from the mostly empty drive.

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post #14 of 15 Old 08-07-2010, 02:02 PM
 
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Yep, so this is why the parity drive needs to be as large as your largest drive.

Taking ericbsmith's example further, using different sized drives and a parity drive smaller than the largest drive. Assume A,B,C are 2GB and Parity is 1GB and all drives are full:

Code:
A   B   C   Parity
0 + 0 + 0 = 0
1 + 0 + 0 = 1
0 + 1 + 0 = 1
0 + 0 + 1 = 1
0 + 1 + 1
1 + 1 + 0
1 + 0 + 1
1 + 1 + 1
Notice the parity drive cannot put in a 1 or 0 for half the data, because it is too small.

Also, the example shows why the number of drives does not matter. If you keep adding more columns, the math simply takes a little longer to do...but the outcome is still only a 1 or a 0.
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post #15 of 15 Old 08-07-2010, 04:12 PM
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Quote:
Originally Posted by a_user View Post

Okay thanks. So in my example this is what I have found.

I have 3.8tb of data across three 2tb data drives and than a dedicated parity drive of 2tb. I copied all my movies (3.8tb) worth none larger than 6gb (have excluded the hd for now). My parity drive has filled to only have 127gb free.

So I guess my question is, is this normal? If a 2tb drive should be able to handle 3 x 2tb drives with full data, I am just about maxed on my parity drive and still have another 1.5tb of space on my data drives.

This is where some of the confusion is coming in.

For your parity drive to only have 127GB free then that means at least one of your 2TB drives only have about 127GB of space remaining. If you moved data from the highest filled data drive to a lower one, you'd see your parity drive size decrease after rsync'ing to about the same size as the highest filled drive.

Just because drive 1 has 2TB of data on it and the parity drive is also full at 2TB does not mean you cannot keep adding data to other data drives with free space. In this scenario, the parity size wouldn't increase, but only change it's value based off the new data that was added to a drive.
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