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post #31 of 53 Old 08-27-2013, 08:32 AM
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Originally Posted by renethx View Post

I think "RAID (Redundant Arrays of Independent Disks)" is still a good term because of "redundancy" by creating parity of a disk "array" (a linked group of disks). Just it does not stripe data across the disks so it does not fall into the traditional RAID category (0-6). Maybe "RAID 4-like RAID without data striping"?

Back when I bought a 1.4ghz Athalon (1999?) with (3) 40TB drives (one for Windows 98, two in RAID0 for video capture) I was taught RAID stands for RANDOM ARRAY OF INEXPENSIVE DRIVES. (If I remember correctly)

It might have been REDUNDANT but I am pretty sure about "Inexpensive" With advances in RAID beyond RAID 0 it changes though...

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post #32 of 53 Old 08-27-2013, 08:48 AM
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Yup, originally "I" meant "inexpensive", later "independent", because individual disk drives used in RAID arrays are not necessarily inexpensive (e.g. today's enterprise-class HDDs are very expensive). But "R" means "redundancy" from the beginning (David A. Patterson, Garth Gibson, and Randy H. Katz: A Case for Redundant Arrays of Inexpensive Disks (RAID). University of California Berkeley. 1988).
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post #33 of 53 Old 08-27-2013, 11:45 AM
 
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Quote:
Originally Posted by renethx View Post

I think "RAID (Redundant Arrays of Independent Disks)" is still a good term because of "redundancy" by creating parity of a disk "array" (a linked group of disks). Just it does not stripe data across the disks so it does not fall into the traditional RAID category (0-6). Maybe "RAID 4-like RAID without data striping"?

How about RAID 4-NS (Non-Striping)? I like it.
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post #34 of 53 Old 08-27-2013, 12:27 PM
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Can you guys explain in MORE DETAIL about "redundancy" and "parity "?
Thank you so much.
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post #35 of 53 Old 08-27-2013, 01:37 PM
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Quote:
Originally Posted by Elpee View Post

Can you guys explain in MORE DETAIL about "redundancy" and "parity "?
Thank you so much.

Paypal me $25 I will tongue.gif

I will when I get home later... on my phone now. PM me to remind me

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post #36 of 53 Old 08-27-2013, 01:38 PM
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Quote:
Originally Posted by Elpee View Post

Can you guys explain in MORE DETAIL about "redundancy" and "parity "?
Thank you so much.

Redundancy is simply using more than one disk to store data. RAID 1 is 100% redundant as an exact block level copy is maintained on a second drive. Parity is a way to have redundancy without having to maintain exact copies of the data (bits). A parity bit is spread across three or more drives, such that any one drive may fail (RAID 5). As a result RAID 5 is (n-1)*.9 usable capacity. RAID1 is n/2*.9 usable capacity.
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post #37 of 53 Old 08-27-2013, 04:10 PM
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Quote:
Originally Posted by Elpee View Post

Can you guys explain in MORE DETAIL about "redundancy" and "parity "?
Thank you so much.

Let's play a game: I give you three slips of paper and two large numbers to remember. We'll call them A and B. The numbers are so big that even with your tiniest handwriting, you can only fit one number per slip of paper. After you have prepared, I will randomly remove one of your slips of paper and ask you to read both numbers back. If you had four slips, it would be easy-- just make two copies of both numbers, but with only three slips, you need to use a trick. Write down the following:
1) A
2) B
3) the sum of A and B

This way, no matter what slip I take, you are still left with enough information to compute both A and B. This general solution is extensible to more than 2 numbers:

1) A
2) B
3) C
4) D
5) A+B+C+D

Again, as long as I remove no more than one slip from your pile, you can always recalculate all the numbers. What if I tell you I'm going to take more than one slip away?

1) A
2) B
3) C
4) D
5) A+B+C+D
6) A+2B+3C+4D

No matter which two slips I take, you can always reconstruct everything with a little algebra. This pattern can extend as far as you want. If I were to take 3 slips, you'd need an additional slip with A+3B+5C+7D to keep up. These additional slips of paper act like parity drives. If there is a failure, parity drives let you keep your system running, albeit with degraded performance owing to the additional calculations needed. The actual implementation of parity calculation uses slightly different math, but the calculations are simple and, more importantly, relatively fast.


'Redundancy' means the number of extra drives you are using beyond the bare minimum needed to hold your data. Some people express it as a ratio between those two, other people just state the number of extra drives. Some people use the word 'tolerance', and some people will tell you that one word means the ratio and the other word means the count. Don't worry about the semantics; it's all the same concept.
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post #38 of 53 Old 08-28-2013, 02:46 AM
 
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The difference between a normal parity setup (such as RAID 5) and the parity solutions you see most people here using (disParity, FlexRaid, SnapRaid) is that the normal parity setup will put a piece of the parity on each drive while the solution like disParity will put all the parity onto one drive reserved for such a purpose. Here are some pictures to illustrate:

This is RAID-5, which is very commonly used in businesses. The sub 1, sub 2, and sub 3 are pieces of the data (think of it as splitting the file into thirds and putting one third on each drive - not exactly correct but close enough) and the sub p is the parity info. 1 + 2 + 3 = p



Unfortunately, I could not find a picture of the parity solution most people here use. But instead of having the parity info (the sub p blocks) spread over all the drives it is all placed onto one drive and the data is not cut into pieces and put over all the drives (called striping) but is all contained on a single drive like how a HDD normally works. The downside to it is that it is slower than a normal RAID solution but the upside is that you can easily add and remove drives from the setup without killing the entire array (you just have to make the change in the controlling software and do a recomputed of the parity).
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post #39 of 53 Old 08-28-2013, 03:24 AM
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post #40 of 53 Old 08-28-2013, 04:31 AM
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If you want to be completely precise, RAID4 does not properly describe FlexRAID etc, since RAID4 still does block-level striping, while FlexRAID for example does not do that at all, it just computes parity over arbitrary data and stores it on a dedicated parity disc - so the default RAID4 level does not fit 100% either.

I suppose for demonstration purposes the image works, though.
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post #41 of 53 Old 08-28-2013, 05:06 AM
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post #42 of 53 Old 08-28-2013, 09:03 AM
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I'm not good at making pictures but from what I understand SnapRaid/ FlexRaid FlexRAID computes parity bit and bulds it on a dedicated parity drive as well as restores a damaged data drive like this

- Build parity drive: If the first bit of data drive 1 is '0' and first bit of data drive 2 is '0, the first bit of parity drive will be added '1' and so on.. (see below)

Data Drive 1 Data Drive 2
> Parity Drive
0 0
> 1
1 1
> 1
0 1
> 0
1 0
> 0
- Restore data drive: Let say we have data drive 2 failed and a new drive was just replaced. If the first bit of data drive 1 is '0' and the first bit of parity drive is '1', the first bit of restoring data drive 2 MUST be added '0' and so on...

Data Drive 1 Parity Drive Data Drive 2
0 1
> 0
1 1
> 1
0 0
> 1
1 0
> 0


That's why Andrea (SnapRaid) and Brahim (FlexRaid) required the parity drive must be the biggest drive in the array.
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post #43 of 53 Old 08-28-2013, 02:01 PM
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Not biggest drive, only must be equal to your largest data drive.

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post #44 of 53 Old 08-29-2013, 01:00 AM
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post #45 of 53 Old 08-29-2013, 05:05 AM
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Quote:
Originally Posted by renethx View Post

biggest = largest ?

The parity drive must be equal in capacity to your highest capacity data drive.

It does not need to be the "biggest"

Example: I have 12 data drives of 3TB. My parity drive is also 3TB.

The words largest and biggest are not good terms. We are really talking about HDD capacity with these terms. But saying biggest suggests the parity drive must be larger than the data drives. This is not true.

You can make a flexraid parity array out of all the same drives. Most do this.

If you have different capacity drives then any size can be used for data but your parity drive must be equal to your largest capacity data drive.

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post #46 of 53 Old 08-29-2013, 05:23 AM
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Quote:
Originally Posted by Mfusick View Post

Quote:
Originally Posted by renethx View Post

biggest = largest ?

The parity drive must be equal in capacity to your highest capacity data drive.

It does not need to be the "biggest"

Example: I have 12 data drives of 3TB. My parity drive is also 3TB.

The words largest and biggest are not good terms. We are really talking about HDD capacity with these terms. But saying biggest suggests the parity drive must be larger than the data drives. This is not true.

You can make a flexraid parity array out of all the same drives. Most do this.

If you have different capacity drives then any size can be used for data but your parity drive must be equal to your largest capacity data drive.

Well, the parity drive can be larger than the highest capacity data drive, not necessarily equal to the highest. smile.gif

Mathematically the biggest of 12 3TB drives is just 3TB. Max (3, 3, ..., 3) = 3. (E.g. Excel max function and many others.)

SnapRAID manual:
Quote:
As parity disks, you have to pick the biggest disks in the array, as the redundancy information may grow in size as the biggest data disk in the array.

That's the normal usage of "biggest" = "largest" in science and engineering.
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post #47 of 53 Old 08-29-2013, 05:24 AM
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Quote:
Originally Posted by Mfusick View Post

The parity drive must be equal in capacity to your highest capacity data drive.
Totally agreed.
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post #48 of 53 Old 08-29-2013, 05:47 AM
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Quote:
Originally Posted by Mfusick View Post

The parity drive must be equal in capacity to your highest capacity data drive.

The parity drive must be as big as your biggest data drive OR bigger. smile.gif

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post #49 of 53 Old 08-29-2013, 06:00 AM
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That's right. I am wondering why Mfusick and Elpee think it must be equal to the highest capacity drive.
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post #50 of 53 Old 08-29-2013, 06:12 AM
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Quote:
Originally Posted by RobNY View Post

The parity drive must be as big as your biggest data drive OR bigger. smile.gif

-RobNY

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post #51 of 53 Old 08-29-2013, 07:25 AM
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Quote:
Originally Posted by renethx View Post

That's right. I am wondering why Mfusick and Elpee think it must be equal to the highest capacity drive.

biggrin.gif

Just like I took it literally it must be bigger and not just equal you took it as it must be equal and not bigger

It must be equal or greater in capacity biggrin.gif

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post #52 of 53 Old 08-29-2013, 12:57 PM
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Technically, the parity drive does not have to have capacity equal to or larger than the capacity of the largest data drive (at least with snapraid). It only needs to have capacity equal to or larger than the amount of data on the data drive with the most data. Of course, if you had a 4TB data drive (with say, 2.5TB of data) and a 3TB parity drive, you would have problems later if you added more than 0.5TB of data to the 2.5/4TB data drive.
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post #53 of 53 Old 08-29-2013, 01:09 PM
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Quote:
Originally Posted by jim2100 View Post

Technically, the parity drive does not have to have capacity equal to or larger than the capacity of the largest data drive (at least with snapraid). It only needs to have capacity equal to or larger than the amount of data on the data drive with the most data. Of course, if you had a 4TB data drive (with say, 2.5TB of data) and a 3TB parity drive, you would have problems later if you added more than 0.5TB of data to the 2.5/4TB data drive.

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