A few questions about RAID 5

[Zucarita9000]

I have a server that has 3 disks, and I'm planning on setting up a hardware RAID 5 configuration. Since RAID 5 requires a 3 disk minimum, if 1 disk fails, does the RAID 5 "dissappear" and the server stops working, or does it keep running long enough for a replacement drive to be put in for rebuilding? Is this allowed for hot swappable drives?

The server is a brand new ProLiant ML350 with three 146GB 10k SAS drives on a hardware RAID controller. The drives are hot swappable.

I have setup RAID0 and RAID1 before, but never RAID5. Let's say one of the drives fails, what would happen? Will I be able to hot swap a drive and keep on going? Should I get a few more drives before setting up the array or is it possible to add more drives afterwards?

 

[CalvinHobbes]

Raid 5:

Striped set with distributed parity or interleave parity. Distributed parity requires all drives but one to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive.

I've never tried to expand a RAID 5 array by adding additional drives so I don't know if that would work. It should be possible but I would imagine the array would need to be rebuilt.

 

[Stump1000]

What happens if you get your new replacement drive and you can't get any of the old information off of the old drive?

 

[CalvinHobbes]

Originally posted by: Stump1000
What happens if you get your new replacement drive and you can't get any of the old information off of the old drive?

The data is rebuilt from the remaining drives when the new drive is added to the array.

 

[Zucarita9000]

Ok. Thanks. Now I need to know if the array can be expanded without much trouble.

 

[yinan]

It all depends on the controller. Some allow you to do on the fly expansion some dont.

 

[Zucarita9000]

Originally posted by: yinan
It all depends on the controller. Some allow you to do on the fly expansion some dont.

It's an HP SmartArray E200i
Link

I'm trying to find out if supports on the fly expansion.

 

[Tarrant64]

Do you know how well it will work for RAID 5 since it doesn't support it natively?

 

[Zucarita9000]

Originally posted by: Tarrant64
Do you know how well it will work for RAID 5 since it doesn't support it natively?

What do you mean? It says it supports RAID 5 with the battery option.

 

[Tarrant64]

Originally posted by: Zucarita9000

Originally posted by: Tarrant64
Do you know how well it will work for RAID 5 since it doesn't support it natively?

What do you mean? It says it supports RAID 5 with the battery option.

Sorry, maybe I misunderstood it. It just looked like it didn't support RAID 5 without an added option so I was wondering if it performs the same as another controller that has it built-in already. That's all.

 

[Zucarita9000]

Oh, that's ok. I have both the BBWC option and the 128MB upgrade so it should support RAID 5.

 

[imported_wired247]

that sounds like a pretty good price for what it does. Although I think you will see significantly better performance if you use 4 drives instead of 3.


edit: oops

 

[BonzaiDuck]

Originally posted by: wired247
that sounds like a pretty good price for what it does. Although I think you will see significantly better performance if you use 4 drives instead of 3.


edit: oops

I can verify this point. The more, the better, but it means more power consumption. I chose four drives for my bottom-of-the-line 3Ware 9650SE 4-port model.

 

[Zucarita9000]

Originally posted by: BonzaiDuck

Originally posted by: wired247
that sounds like a pretty good price for what it does. Although I think you will see significantly better performance if you use 4 drives instead of 3.


edit: oops

I can verify this point. The more, the better, but it means more power consumption. I chose four drives for my bottom-of-the-line 3Ware 9650SE 4-port model.

I've order two more SFF SAS drives, so I can use 4 for the RAID array and have a spare in the case of a drive failure.

Thanks a lot for your help.

 

[Zucarita9000]

Ok, I've found this:

Data Compatibility among all models of Smart Array controllers allows simple and easy upgrades
any time needs for higher performance, capacity, and availability increase. Even successive
generations of Smart Array controllers understand the data format of other Smart Array
Controllers.

Seamless upgrades from past generations and upgrades to next generation HP high
performance and high capacity Serial Attached SCSI Smart Array controllers.

Addition of the battery backed cache upgrade enables BBWC, RAID 5, Capacity Expansion,
RAID migration, and Stripe Size Migration.

So apparently the BBWC upgrade that I purchased was worth it. It enables RAID5 AND capacity expansion.

 

[Madwand1]

Originally posted by: Zucarita9000
I have a server that has 3 disks, and I'm planning on setting up a hardware RAID 5 configuration. Since RAID 5 requires a 3 disk minimum, if 1 disk fails, does the RAID 5 "dissappear" and the server stops working, or does it keep running long enough for a replacement drive to be put in for rebuilding? Is this allowed for hot swappable drives?

Good questions. I'm sure there are many more that you can come up with -- e.g. does the drive ordering matter? How exactly do I grow the array? What happens if I lose power in the middle of a re-build? What happens if I reinstall the OS? What's the best stripe size for performance?

Some of this is specific to the controller you have and can't be simply generalized from other RAID 5 implementations. The best time for you to learn this is before you have a lot of real data committed to the array. I suggest playing around with the array and testing failure cases to see these things for yourself; to learn things you didn't think to ask about ahead of time; to learn from experience while it's not going to cost you real data loss.