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Suppose Server 1 is configured with 8 1TB disks in RAID 5. Server 2 is configured with 8 1TB disks in RAID 0. There is a link between the two servers but for the purpose of this question, lets suppose that the link speed is unlimited. I want to copy data from Server 1 to Server 2.

Please advise which of the below statements would be true:

(Presuming all hardware is identical. The only difference is the RAID configurations).

Statement 1

Movement of data works like water flowing through a pipe. If the pipe is thin at one end and wider at the other end, the water will move slowly at one end therefore, the other end would not benefit from its wider size. This is the same as copying data from RAID 5 to RAID 0. RAID 0 has better performance however, it does not benefit from it because there is a bottleneck with the RAID 5 server.

Statement 2

Suppose Server 1 is a man with a book and Server 2 is another man with another book. If Server 2 (the second man) can read and write quickly, he can look at the first man’s book to rapidly read all of its contents and then write it down in his own book regardless of how fast (or slow) the first man can read and write.

Paul Gunston
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2 Answers2

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RAID 0 shouldn't be used. There's enough information out there detailing RAID levels that I don't think you need to benchmark this. I hope this isn't a homework question.

What are the different widely used RAID levels and when should I consider them?

Community
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ewwhite
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Neither statements are exactly true, the first one is closer.

In computer science we have this term that we call bottle neck. It comes from something like your first example. The speed at which you can fill a bottle with liquid depends on how thin is the neck of the bottle.

The bottle neck could usually be exactly established and pin-pointed to a very concrete component. You always start from the big ones.

You have 2 servers and a link. You determine which one has the slowest action:

  • Server A is reading data. What is the maximum speed?
  • Server B is writing data. What is the maximum speed?
  • The link is transferring data. What is the maximum speed?

Now you determine what is the lowest transfer speed from the 3 above and, for example, you determine that writing data to Server B is slower both than reading data on Server A and transferring data through the link between them. You start looking at the possibilities:

  • The CPU of server B is really slow and cannot cope with the traffic. Highly unlikely.
  • The BUS between the RAM and CPU is really slow and cannot cope with the speed. Also quite highly unlikely.
  • Etc...
  • Most likely scenario: either the controller or the disks themselves have the lowest transfer speed. This could include stuff like the data sectors are not aligned between the filesystems, logical volumes, partitions, RAID configuration etc.

There is no direct connection between what RAID configuration you are using on each server as each of them are operating completely unrelated to the other.

Florin Asăvoaie
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