RAID Configurations

[Matt Midgley]

Hi All,

Just a question or two on RAID configurations. I've been looking at the Buffalo TeraStation Pro II, I like it!!

First of all, I'm trying to establish if it will support NTFS. If it doesn't, it's pretty useless to me. Also, it supports 4 HDD's. Here's the main bit:

Would the TeraStation (or any other NAS solution) allow you to use RAID 1 with four disks so that:

Disk 1 (250/500GB) and Disk 2 (250/500GB) Total: 500GB/1TB

are mirrored to:

Disk 3 (250/500GB) and Disk 4 (250/500GB) Total: 500GB/1TB

or would I have to do it in pairs like this:

Disk 1 (250/500GB) mirrored to Disk 2 (250/500GB) Total: 250/500GB
Disk 3 (250/500GB) mirrored to Disk 4 (250/500GB) Total: 250/500GB

meaning I would end up with my storage solution would be split into two logical drives (C:\ and D:\). If the first solution would work it would be great, I could have a single logical drive with redundancy.

Hope you can make sense of this!

 

[Matthew]

Hello Matt,
I am not sure on NTFS but I would expect it would be formatted that way. I personally would more then likely go for the RAID 5 parity so that you get 750Gb out of the 1Gb of storage and configure it as 1 virtual drive.


We run a few different configs in the office. Let me know what your thinking of and I will try get the best for you.

Another way we run in the office is to have 3 drives and then 1 hot spare which kicks in should a drive fail.

 

[Matt Midgley]

Matt, thanks for the reply, sorry for my delay in response. I don't need that much storage yet, but I can see it in the near future with all the digital photos and movies I take. In short, I'd like a large ammount of redundant storage. The hardware would be worth nothing in comparison to the photos and movies!!!

Can you shed more light on Raid 5 parity? I've heard about it and read up on it but I can't quite grasp it/how it works. Does it provide a high level of redundancy like mirroring?

Also, I've read up more on the file systems. It turns out that it'll store files larger than 4GB. I think it's only FAT32 with that limit. It doesn't support NTFS but it does support some other Linux file systems that have size limits of several terabytes.

Ta,
Matt

 

[Matthew]

From what I understand, data is copied to the drives. If you have 4 drives then data is written to 3 of the drives and the 4th drive contains all the data of that particular block and is called the parity block.

When the next data is written then 3 of the drives have the data distributed and the 4th drive has all of the data.

The parity blocks are distrubuted as below...

example...

X1, X2, X3, Xp - X1 is on drive 1, 2 on drive 2, 3 on drive 3 and parity on drive 4.

Y1, Y2, Yp, Y3 - Y1 is on drive 1, 2 on drive 2, parity
on drive 3 and 3 on drive 4.

Z1, Zp, Z2, Z3 - Z1 is on drive 1, parity is on drive 2, 2 is on drive 3 and 3 is on drive 4.

As you see above the parity block of each set of data is put on another drive. Should drive 4 fail then the data Y3 and Z3 which would be lost, can be re created from Yp on drive 3 and Zp on drive 2.

RAID 5 can speed up drive reads should blocks need to be read at the same time, however, if 2 blocks such as Y3 and Z3 need to be read at the same time then one will have to wait a few ms.

It's cool to work with in a high uptime environment. We have a server in the office here that is used by people across Europe who log data, transfer FTP files etc... When a drive failed a few months back I just put a new drive in and it rebuilt. I didnt even switch the server off, reboot or anything and none of the users knew except those who spotted me in the office replacing a drive.

 

[Matt Midgley]

Thanks for the info Matt. That's exactly what I need. Sounds loads simpler than mirroring. So, this configuration can handle a single drive failure? That's not bad, especially in a home network use environment. I think the chances of two drives failing are slim enough!

 

[Matthew]

It can handle a single drive failure. If you run it with a hot spare then I believe it can handle 2 drive failures.

Funnily enough I have seen 1 instance of 2 drives dieing... and also seen a raid controller die. Both lost everything. The company had backups, but they were TBs in size... it sucked restoring them and took all weekend.

 

[Matt Midgley]

...Funnily enough I have seen 1 instance of 2 drives dieing... and also seen a raid controller die. Both lost everything. The company had backups, but they were TBs in size... it sucked restoring them and took all weekend.

Ah, I was about to trust RAID5 more than that. So, if you have a RAID5 array and the controller dies, you can't put another controller on to it and continue using it? Another thing I've seen concerning RAID5 is file corruption. How can you corrupt a file on a RAID5 array? What are the chances of corrupting an entire array?

Thanks

 

[Matthew]

I am not sure to be honest. The RAID controller we had was several years old, so the newer ones could be a little more tollerant to problems. I havent seen any file corruption on any of the servers we have here, so I am not sure how much of a problem that can be. Regarding the whole array corrupting... it happened here, so I guess it does happen