It would appear to me performance wise, they are the same, isn't it true?
That is, if we write a 1GB file to Raid 10 or Raid 01, then we are letting each drive write 500MB of data, so it is increased speed for either case.
Is it true that it is only for fault tolerance and how the controllers are designed?
For example, if there are 4 drives total, and if Drive 1 fails:
- in Raid 10, then 3 drives will still be working hard for you. When Drive 1 fails in a mirrored situation, Drive 2 will still keep on working hard. Drive 3 and 4 are also working hard. Only 25% of the setup is down.
- in Raid 01, only 2 drives will now be working hard for you (Drive 3 and 4). In fact, you may as well turn off Drive 1 and Drive 2, because the way it is designed, the controllers will not write part of the data to Drive 2 like it writes to Drive 4. Both Drive 1 and 2 will not be used at the same time. 50% of the setup is down.
It is even more apparent in a 12 drive situation. If Drive 1 fails:
- In Raid 10, then 11 drives will still be working for you. 91.66% of the setup is still up and running.
- In Raid 01, then 6 drives will all stop running at the same time. Again, 50% of the setup is down.
Note that this is how Raid 1 (mirror) is designed: if one drive is down, it will keep busy with the remaining drive. In Raid 0 (striped), if one drive is down, the whole series of drives will all stop working.
And imagine if Drive 1 fails in Rail 01, and you need to wait 5 days to get a new drive to rebuild. If during that 5 days, any one of Drive 7 - 12 dies, then your system is in a really bad shape. Because Drive 7 - 12 is constantly changing its content. If Drive 12 dies, can you use Drive 6 to substitute for it? You cannot, because Drive 12 and 6 are different already.
But if it is Raid 10 and Drive 1 fails, and during that 5 days you wait for a new drive, if Drive 3 - 12 fails, that's fine. You still have everything. It has to be exactly Drive 2 that dies that the system will be in a bad shape, which is less likely to happen than any one of Drive 7 - 12 failing.
And actually if in Raid 01, when rebuilding 5 days or even just one day later, don't we have to rebuild Drive 1 to 6, because Drive 7 - 12 have already changed enough, so Drive 2 to 6 are also out of sync? In Raid 10, all that needs to rebuild is only one drive.
Is that exactly the reason why? Performance wise, it is all the same. It is about tolerance that matters that makes Raid 10 better than Raid 01?