What happens if the parity drive fails?

[STiCory]

Sounds like a simple question, but it it is one that I have never known the answer to. For example RAID 5 on four disks, 1-3 are the striped data, and 4 is the parity disk, what happens if the parity disk fails? Can the RAID controller rebuild the parity drive off of striped data on disks 1-3? Even harsher example, RAID 6 on 5 drives, 1-3 is data again, 4 and 5 are parity disks, what happens if both 4 and 5 go down? Can a rebuild occur? What if for example 1 and 3 go down? Some things that I have always wondered, and everyone always just says "In RAID 5 one disk can go down and in RAID 6, two disks can go down"

 

Sorry this is my first post, new to the forums, but I have been watching LTT for a long time, just never thought of actually using the forums for some reason lolol.

[Skiiwee29]

2 minutes ago, STiCory said:

Sounds like a simple question, but it it is one that I have never known the answer to. For example RAID 5 on four disks, 1-3 are the striped data, and 4 is the parity disk, what happens if the parity disk fails? Can the RAID controller rebuild the parity drive off of striped data on disks 1-3? Even harsher example, RAID 6 on 5 drives, 1-3 is data again, 4 and 5 are parity disks, what happens if both 4 and 5 go down? Can a rebuild occur? What if for example 1 and 3 go down? Some things that I have always wondered, and everyone always just says "In RAID 5 one disk can go down and in RAID 6, two disks can go down"

 

Sorry this is my first post, new to the forums, but I have been watching LTT for a long time, just never thought of actually using the forums for some reason lolol.

Yes, all the data with a parity bit will be rebuilt by the array. That is what a parity bit is for. In the event of a failure, you can put in a new drive and the array should automatically rebuild the missing data based on the parity bit. Now in things like Raid 0 where there is no parity bit, you would have lost everything with no chance of recovery. 

[Skiiwee29]

4 minutes ago, STiCory said:

Sounds like a simple question, but it it is one that I have never known the answer to. For example RAID 5 on four disks, 1-3 are the striped data, and 4 is the parity disk, what happens if the parity disk fails? Can the RAID controller rebuild the parity drive off of striped data on disks 1-3? Even harsher example, RAID 6 on 5 drives, 1-3 is data again, 4 and 5 are parity disks, what happens if both 4 and 5 go down? Can a rebuild occur? What if for example 1 and 3 go down? Some things that I have always wondered, and everyone always just says "In RAID 5 one disk can go down and in RAID 6, two disks can go down"

 

Sorry this is my first post, new to the forums, but I have been watching LTT for a long time, just never thought of actually using the forums for some reason lolol.

Here is a wiki link that will explain it.

 

https://en.wikipedia.org/wiki/Parity_bit

[STiCory]

15 minutes ago, legacy99 said:

Yes, all the data with a parity bit will be rebuilt by the array. That is what a parity bit is for. In the event of a failure, you can put in a new drive and the array should automatically rebuild the missing data based on the parity bit. Now in things like Raid 0 where there is no parity bit, you would have lost everything with no chance of recovery. 

 

14 minutes ago, legacy99 said:

Here is a wiki link that will explain it.

 

https://en.wikipedia.org/wiki/Parity_bit

Holy cow, my understanding of RAID with parity was totally wrong, somehow I thought the parity information was held on a separate drive. I just found an article that explains it really well here. I feel stupid for even wasting the bandwidth on this topic 

[Skiiwee29]

1 minute ago, STiCory said:

 

Holy cow, my understanding of RAID with parity was totally wrong, somehow I thought the parity information was held on a separate drive. I just found an article that explains it really well here. I feel stupid for even wasting the bandwidth on this topic 

lol no problems. It can be confusing with all the different raids available, striping, etc. Thats why the LTT is one of the greatest communities out there as we will actually give you the answers you need to better inform you instead of troll you. Its just how we roll. 

[STiCory]

23 minutes ago, legacy99 said:

lol no problems. It can be confusing with all the different raids available, striping, etc. Thats why the LTT is one of the greatest communities out there as we will actually give you the answers you need to better inform you instead of troll you. Its just how we roll. 

Awesome, looks like I'll be here to stay then! Thanks for the help!

[scottyseng]

9 hours ago, STiCory said:

 

Holy cow, my understanding of RAID with parity was totally wrong, somehow I thought the parity information was held on a separate drive. I just found an article that explains it really well here. I feel stupid for even wasting the bandwidth on this topic 

Some specialized software RAID will actually have one drive as a parity drive, or as many as you want as a parity only drive.

 

For your normal RAID though, yeah, the parity RAID levels have the parity built into the information (Why it takes quite a bit of time to build a parity based array).

[STiCory]

40 minutes ago, scottyseng said:

Some specialized software RAID will actually have one drive as a parity drive, or as many as you want as a parity only drive.

 

For your normal RAID though, yeah, the parity RAID levels have the parity built into the information (Why it takes quite a bit of time to build a parity based array).

Yeah normal RAID 5 or RAID 6 sounds the best, after doing a good bit of research it turns out I was describing RAID 2. RAID 6 sounds definitely like the way to go.

[Electronics Wizardy]

10 hours ago, STiCory said:

Sounds like a simple question, but it it is one that I have never known the answer to. For example RAID 5 on four disks, 1-3 are the striped data, and 4 is the parity disk, what happens if the parity disk fails? Can the RAID controller rebuild the parity drive off of striped data on disks 1-3? Even harsher example, RAID 6 on 5 drives, 1-3 is data again, 4 and 5 are parity disks, what happens if both 4 and 5 go down? Can a rebuild occur? What if for example 1 and 3 go down? Some things that I have always wondered, and everyone always just says "In RAID 5 one disk can go down and in RAID 6, two disks can go down"

 

Sorry this is my first post, new to the forums, but I have been watching LTT for a long time, just never thought of actually using the forums for some reason lolol.

- Raid 5 doesn't have a deticated parity drive, like how raid 4 does. The parity is spread out among all the drives. If you lose a parity bit you can recreate it as its made from the differences from the other data drives.

[Yamoto42]

Some quick advice on Raid5...don't.
 

If you use a parity based system, always use at least double parity.

[STiCory]

15 hours ago, Electronics Wizardy said:

- Raid 5 doesn't have a deticated parity drive, like how raid 4 does. The parity is spread out among all the drives. If you lose a parity bit you can recreate it as its made from the differences from the other data drives.

 

25 minutes ago, Yamoto42 said:

Some quick advice on Raid5...don't.
 

If you use a parity based system, always use at least double parity.

RAID 6 sounds like the way to go for sure!

[jowdemanne]

12 hours ago, Yamoto42 said:

Some quick advice on Raid5...don't.
 

If you use a parity based system, always use at least double parity.

Couldn't agree more! I think the chance of a drive error during a raid 5 rebuild is like 15%, which is pretty damn high. Always use double parity if you have big drives, which is almost always :-)