Storage question - Can anyone explain RAID 5 parity in human terminology?

ande0255ande0255 Posts: 1,178Banned
So I'm running over the storage chapter of Mastering vSphere, and I know it's not a big detail for exam day, but if anyone knows this I would love some clarification for the sake of knowledge (what we at TE live for!!).

It took a bit of time to understand the difference between RAID 1+0 and 0+1, but now I'm onto RAID 5, and after extensive google-fu and reading all sorts of articles including wikipedia on RAID 5 / Parity / XOR calculations I cannot understand what is happening on a RAID 5 array of disks.

Some explanations / diagrams make it seem like a parity bit is written across each disk, some articles made it sounds almost like a Boolean AND'ing only with 1 and 0's representing evens and odds, and in either of those cases I'm not sure what is being written to the parity disk itself that can be deciphered back into usable data.

Is there any simple way of explaining what RAID 5 is? Not sure if I am just tired from reading and my brain is just not accepting any more storage related input for the night, or if these articles I am reading are people with existing knowledge of storage talking on a higher level that I am currently at.

Thanks for whoever reads between refreshing the ESXi 6 thread and drooling over new VMware stuffs icon_thumright.gif
Back in my day we used to route packets on 56k lines, through the snow, uphill both ways.


  • netsysllcnetsysllc ■■■■□□□□□□ Posts: 479Member ■■■■□□□□□□
    Raid 5 stripes the data across each of the disks, but uses part of each disk as parity for information for each of the other disks in a reserved area. So lets say you have a 3 disk RAID 5 array. The third disk dies. Your data is (hopefully) still there because disk 1 and 2 both have part of the information for disk 3, when you replace the drive it will rebuild the data from what is on disk 1 and 2.
  • ande0255ande0255 Posts: 1,178Banned
    What confuses me, is the vSphere book makes it sound like there is a dedicated disk in the array called a Parity Disk, that is having this parity information written to it. These are a couple quotes from the book regarding RAID 5:
    RAID 5 calculates the parity across the drives in the set and writes the parity to another drive.
    When a drive fails in a RAID 5 set, I/O can be fulfilled using the remaining drives and the parity drive, ...

    To further confuse things, the Figure in the book demonstrating a RAID 5 disk array shows the first four disks with bits being written to them, and disk 5 has Parity being written to it. So I'm wondering if you're losing a drive with RAID 5, or if the data is actually striped across all disks and all disks remain usable.

    I was jamming through storage until I hit this road block, I really appreciate the input, and big thanks for your response netsys!
    Back in my day we used to route packets on 56k lines, through the snow, uphill both ways.
  • swefredswefred ■□□□□□□□□□ Posts: 13Member ■□□□□□□□□□
  • LauraMalaveLauraMalave ■■□□□□□□□□ Posts: 52Member ■■□□□□□□□□
    RAID.EDU has nice animations to help visualize the different RAIDs.
  • ande0255ande0255 Posts: 1,178Banned
    Thanks so much for the input, very helpful links!
    Back in my day we used to route packets on 56k lines, through the snow, uphill both ways.
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