New to RAID

[zylander]

I'm setting up a new file server using hardware raid 5 and I have a couple questions;

1. When looking at controller cards, how do I know if the card allows you to expand the array without rebuilding it? What is the technical term?

2. When building an array can you only use one card per array? If I want 16 drives total, do I need to have a card that supports 16 drives or can I use two 8 channel cards?

 

[pjkenned]

1. Companies call these different things. Online capacity expansion, and etc. If you buy a decent LSI/3ware/Adaptec/Areca card, this won't be an issue. I'm pretty sure even ICH10R raid can do it.
2. Generally yes, however you can always software raid two 8 port controller arrays. Realistically though, you are better off on a 16 port one card array. Lower power consumption, less BBU's to worry about, and for your hotspare(s) you have them on a single card.

Other note: At 16 drives, you may want to think about raid 6. Also, I'm going to guess you are using 1TB+ drives, so you might want to think about a hotspare too. Even using dual core IOP 348 controllers, the 10TB+ arrays take a long time to rebuild after a drive failure.

 

[jkresh]

for 16 I would definitely go with raid 6, any good card that can handle raid 5 in hardware can do 6, and while it adds a little to the cost (1 less drives worth of capcity) and slows it down a little, it does add a level of safety as even with 5, with 16 drives there is a not insignificant chance that a 2nd could fail during a rebuild.

 

[RebateMonger]

If you are thinking of creating striped redundant arrays with anywhere near 16 disks, I'd do some research on the concerns about read errors or total disk failures during array rebuilds. These are especially worrisome if you are thinking of using "big" disks (like TeraByte disks).

Basically, the concern is that, with a large number of large disks, the chance of a multiple-disk failure during a rebuild is pretty high.

Here's a starting point to at least get an idea of what the controversy is about.

Also, if you haven't already planned something, consider whether you are going to want backups and how you are going to do that. Backups of large arrays is becoming a serious problem unless you have lots of money. Offsite backups are even tougher.

 

[zylander]

I re-evaluated my plans and Im probably looking at no more than 8 1TB drives in my server.

How does a hotspare drive work? From what Ive read it reduces the time it takes to rebuild the array but is a hotspare drive not used for storage? For example; a RAID 6 array with 8 1TB drives has 6TB of storage. If I use one of those drives as a hotspare, do I still have 6TB's of storage or less?

 

[RebateMonger]

Originally posted by: zylander
How does a hotspare drive work? From what Ive read it reduces the time it takes to rebuild the array but is a hotspare drive not used for storage? For example; a RAID 6 array with 8 1TB drives has 6TB of storage. If I use one of those drives as a hotspare, do I still have 6TB's of storage or less?

Hotspares are NOT used for storage. If the RAID controller detects a drive failure, it automatically begins rebuilding the array using the existing data and the parity information. The controller uses the hotspare to replace the failed disk.

Hotspares can get the rebuild started sooner than waiting for the administrator to switch out a failed drive. With eight 1 TB drives (including a hotspare) in RAID 6, you'll have a storage capacity of 5 TB.

 

[Emulex]

i'd do raid-10 man.

raid-5 blows for write speed. and rebuilding a 1TB drive will take forever. seriously huge amount of time.

if the controller has no battery back write cache - skip.

raid-6 and raid-5 require a double read and write every time you write to the disk.

raid-10 (4gb from 8 drives) does not.

make sure you do not crash or lose power to your storage server as well. that can fail the raid for sure.

I run redundant power to separate UPS's.


Real raid controllers will activate after 3-15 (configurable) seconds to sweep the drives for errors and handle them proactively.

Also do not use consumer drives. RE4 WD, Seagate ES/NS.

Consumer 1TB drives + raid-5 = guaranteed recipe for disaster.