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Thread: The knowledge exchange

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    The knowledge exchange

    I have been pondering where to put this, and decided that Science&Tech made the most sense, but really that was based on what I considered contributing.

    Basically, I like to acquire knowledge on things, whether I find they will serve me in life or not, and I also find some pleasure in teaching things to people who may think a topic or a specific piece of information is beyond their ken. Most of the time I find that a person's inability is because they just haven't heard it explained in a way they can process.

    I am an IT nerd, so most of what I will mostly keep my quick lessons to that topic. If you have a question where you need something explained, or you have a strong grasp on a subject that you want to teach, add it to this thread... we'll see how this works.

    My test entry coming shortly...
    The only path to truth is paved with skepticism.

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    Re: The knowledge exchange

    Quote Originally Posted by jmotivator View Post
    I have been pondering where to put this, and decided that Science&Tech made the most sense, but really that was based on what I considered contributing.

    Basically, I like to acquire knowledge on things, whether I find they will serve me in life or not, and I also find some pleasure in teaching things to people who may think a topic or a specific piece of information is beyond their ken. Most of the time I find that a person's inability is because they just haven't heard it explained in a way they can process.

    I am an IT nerd, so most of what I will mostly keep my quick lessons to that topic. If you have a question where you need something explained, or you have a strong grasp on a subject that you want to teach, add it to this thread... we'll see how this works.

    My test entry coming shortly...
    I, too, love acquiring knowledge on a variety of subjects. I've always loved the conception "renaissance man". Some of that knowledge is truly obscure. Good luck.
    Mission: "to defend and preserve the individual rights and liberties guaranteed to every person in this country by the Constitution and laws of the United States." ACLU. Why isn't every American a member?

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    Re: The knowledge exchange

    First subject is a cool one to teach because I think a lot of people will here some buzzwords about this but just assume the explanation is beyond them.

    I'm going to explain how Raid Arrays and Disk Parity works.

    RAID stands for "Redundant array of inexpensive disks" where "redundant" implies fault tolerance. There are numerous versions of RAID that you can employ that provide different levels of tolerance and efficiency. Historically RAID was deployed to create a fast logical disk from a group of individual slower disks. For example, and very roughly, if a disk was capable of writing and reading information 30 times a second (known as IOPS, or "Input/output per second), then two disks in RAID configuration would share the load and so the logical disk could write at 60 (again, a simplification)

    Now, if you had a 2 disk array each sharing half the load, what happens is one disk dies? Well, the answer is you lose everything. Since files are written across 2 disks, you would end up with only half the data for each file, and likely not in any logical order. This two disk RAID is known as RAID-0.

    RAID-1 is a form of parrity called "mirroring" which is, quite simply, the creation of two disks wit the exact same data. All IOPS being written to disk are written simultaneously.

    The next logical RAID format is RAID-10.. oddly named since most other raid formats are simply numbered in order of discovery. But RAID-10 is a cute name that denotes a RAID-1 Mirror of two RAID-0 arrays (it is also sometimes referred to as RAID-1+0).

    RAID-4 is where it gets interesting. RAID-4 introduces Parity to a RAID array, and in a RAID-4 array, one disk of the array is dedicated to storing Parity information. What that Parity is is what is so interesting.

    To explain how Raid-4 Parity works, you first need to understand a bit about binary. Binary is a numerical system that is Base 2 where every number is represented by a string of 0s and 1s ... like the Base 10 decimal system, each digit in a binary number denoted a higher order, with the value increasing by 2 rather than 10. So, the number 0 in Binary is 0, 1 is 1.... the second digit in Binary is 2, so the number 2 is 10 (2+0) and the number 3 is 11 (2+1) ... with me so far? OK... back to Parity!

    Now all data in a computer is stored in Binary, so any file, to the computer, is simply a series of 0 and 1 that an application is written to interpret .... in a RAID array any such file is broken down in "blocks" of data, or smaller portions of 0s and 1s

    "Blocks" are, in simple terms, address spaces on a disk, each with a unique address. Each file on a computer is registered in a type of address book (Called a File Allocation Table, aka "Fat Table") on the disk that tells the computer how many blocks the files use, what the address is for each block, and what order they should be read in.

    NOW the fun starts. See, a computer doesn't really care what most of those 0s and 1s in a block actually mean, and deals with data mostly as just 0s and 1s. So RAID just thinks of each block as a binary number (I'll convert to decimal in my explanation for clarity)... RAID then "stripes" data across similarly addressed blocks.. so, for instance, in a 3 disk RAID-1 array, Disk 1-Block 1, Disk 2-Block 1 and Disk 3-block 1 would constitute a "stripe". In a RAID-4 array there would be a forth disk that holds parity data.

    What is parity? Well, it is just Computers doing Sudoku. So, for instance, lets say that the binary number in the stripe on disk 1 happens to translate to the decimal number 27, and 14 on Disk 2 and 40 on Disk 3... the Parity data simply adds 27,14 and 40 and gets 81, and the number 81 is written on the parity disk in binary.

    Now let's say that we lose Disk 2. The computer looks at the block 1 stripe and sees, essentially "27+?+40=81" and from that it deduces the the missing number is 14, and so it reads the binary code for 14 deductively, rather than physically, when reading the file.

    In a System running RAID-4 you would also likely have a Spare disk that, on the failure of Disk 2, would then start populating it's blocks by running through each stripe and deducing the missing block. Eventually the system would rebuild the missing disks so that future reads would be done physically rather than deductively.

    RAID-5 is essentially the same function as RAID-4, but instead of having a single disk housing all Parity data, the parity is distributed across all disks. The Parity functions the same, but you don't essentially lose a full disk of IOPS due to a mostly inactive parity disk.

    RAID-6 is RAID-5 with an additional Parity block with means that, in theory, a RAID-6 array can lose 2 disks without the array dying since all lost blocks (data or parity) will have a stripe by which it can deduce a missing value or two.


    So there you have it. (some info cut to fit in a single post)
    Last edited by jmotivator; 11-26-19 at 04:28 PM.
    The only path to truth is paved with skepticism.

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