On 09/14/2013 04:50 AM, Grant wrote: > > Instead, how about a 6-drive RAID 10 array with no hot spare? My > guess is this would mean much greater fault-tolerance both overall and > during the rebuild process (once a new drive is swapped in). That > would mean not only potentially increased uptime but decreased > monitoring responsibility. >
RAID10 with six drives can be implemented one of two ways, Type 1: A B A B A B Type 2: A B C A B C If your controller can do Type 1, then going with six drives gives you better fault tolerance than four with a hot spare. I've only ever seen Type 2, so I would bet that's what your controller will do. It's easy to check: set up RAID10 with four drives, then with six. Did the drive get bigger? If so, it's Type 2. If it's Type 2, then four drives with a spare is equally tolerant. Slightly better, even, if you take into account the reduced probability of 2/5 of the drives failing compared to 2/6. No one believes me when I say this, so here are all possibilities for a two-drive failure enumerated for four-drive Type 2 (with a spare) and six-drive Type 2. Both have a 20% uh oh ratio. Layout: A B A B S 1. A-bad B-bad A B S -- OK 2. A-bad B A-bad B S -- UH OH 3. A-bad B A B-bad S -- OK 4. A-bad B A B S-bad -- OK 5. A B-bad A-bad B S -- OK 6. A B-bad A B-bad S -- UH OH 7. A B-bad A B S-bad -- OK 8. A B A-bad B-bad S -- OK 9. A B A-bad B S-bad -- OK 10. A B A B-bad S-bad -- OK Layout: A B C A B C 1. A-bad B-bad C A B C -- OK 2. A-bad B C-bad A B C -- OK 3. A-bad B C A-bad B C -- UH OH 4. A-bad B C A B-bad C -- OK 5. A-bad B C A B C-bad -- OK 6. A B-bad C-bad A B C -- OK 7. A B-bad C A-bad B C -- OK 8. A B-bad C A B-bad C -- UH OH 9. A B-bad C A B C-bad -- OK 10. A B C-bad A-bad B C -- OK 11. A B C-bad A B-bad C -- OK 12. A B C-bad A B C-bad -- UH OH 13. A B C A-bad B-bad C -- OK 14. A B C A-bad B C-bad -- OK 15. A B C A B-bad C-bad -- OK
