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Raid 5 - How many drives?

post #1 of 12
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So I'm planning out a build for a file sever, but one thing that I'm not sure about is how many drives to put in my Raid 5 array. I want a balance of cost effectiveness with security. So I'd like to use more than 3 drives (the minimum), but I'm not sure how many I should use before the security of using Raid 5 becomes lost.

Right now I'm considering using 5 drives per array. Is that safe enough? Does anyone here have any experience with Raid 5, and if so how many do you use?
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post #2 of 12
Well, I don't have any experience, but i do know the concept.
RAID 5 (with 5 drives) would use 4 for storage and 1 for parity. If any drive goes out, it can be rebuilt. If a 2nd drive goes out before you get the first one rebuilt, then you lose your data. It only becomes insecure when the time to rebuild one drive is longer than the time it takes for another drive to fail.
5 drives will be PLENTY secure with RAID 5.

If you plan to have 10+ drives in an array, I would probably invest in RAID 6 which allows any 2 drives to fail and still recover your data.

The efficiency of drives/storage goes up with more drives because
RAID 5: storage = smallest drive x (total drives - 1)
RAID 6: storage = smallest drive x (total drives - 2)

Souce
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post #3 of 12
First off, you MUST get a good RAID card if you want to do RAID 5 (And for 5 drives+, that's going to be like $500 for an 8 port card).

I recommend doing the 5 drive RAID 5 + a hot spare (which automatically takes over in the event a drive fails -- it'll rebuild it automatically, to greatly reduce the amount of time where you're at risk for a real failure)

And the security of RAID 5 is NEVER "lost". It ALWAYS Will be more secure then a RAID 0, even if you have 100 drives in the array. It's just more likely that you'll have 2 concurrent drive failures.
post #4 of 12
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Quote:
Originally Posted by demoship
First off, you MUST get a good RAID card if you want to do RAID 5 (And for 5 drives+, that's going to be like $500 for an 8 port card).
I'm planning to do software RAID as the processor will be completely idle otherwise, so wasted cycles is not a hindrance. And then I also avoid said $500 price tag. I know raid cards have some extra benefits (dynamically adding drives to the array, etc), but I can probably go without them.

Quote:
Originally Posted by demoship
And the security of RAID 5 is NEVER "lost". It ALWAYS Will be more secure then a RAID 0, even if you have 100 drives in the array. It's just more likely that you'll have 2 concurrent drive failures.
Well, perhaps I used the wrong term, but I mean "at the point where it becomes likely I will lose data". Seeing as the whole point of doing raid 5 in this situation has nothing to do with performance, only data security.
Quote:
Originally Posted by bruestle2
5 drives will be PLENTY secure with RAID 5.
Quote:
Originally Posted by demoship
I recommend doing the 5 drive RAID 5 + a hot spare (which automatically takes over in the event a drive fails -- it'll rebuild it automatically, to greatly reduce the amount of time where you're at risk for a real failure)
I'll probably go ahead with the 5 drive array then. The hot spare is interesting too, something I didn't think of.

Thanks.
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post #5 of 12
I'd be very cautious about using software RAID 5. I did speak w/ a systems admin a while back, and he said that using software raid w/ more advanced raid arrays can be very dicey. Using software RAID 5 can give you a false sense of security. When a drive actually fails, you might end up losing all your data anyway, even though it was "supposed" to have redundancy.

RAID 5 is also EXTREMELY slow when writing unless you have a dedicated card to it (a good one). It doesn't matter if your CPU is idle, it's just not efficient enough to do the specialized work needed for RAID. (plus every bit you're writing has to go to more places for processing, which takes more time).

If you don't want to shell out the money for real RAID 5, I advise that you use RAID 1 or no RAID.
post #6 of 12
Ummmmmmmmmm, you only need a minimum of 3 drives to use RAID 5 and it does not use 2 for data and 1 for parity it uses all drives storing bits of data and parity among all the drives. The RAID 5 setup is the most commonly used setup for security and performance it allows the user to completely loose 1 hdd while still maintaining all data.

get a tyan mobo or supermicro most server boards come with BIOS supported Raid 0,1,5, and 10 support


RAID 5: Striped Set (3 disk minimum) with Distributed Parity. Distributed parity requires all but one drive 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.


A little more info

* RAID 0: Striped Set (2 disks minimum) without parity. Provides improved performance and additional storage but no fault tolerance from disk errors or disk failure. Any disk failure destroys the array, which becomes more likely with more disks in the array. The reason a single disk failure destroys the entire array is because when data is written to a RAID 0 drive, the data is broken into "fragments". The number of fragments is dictated by the number of disks in the drive. Each of these fragments are written to their respective disks simultaneously on the same sector. This allows smaller sections of the entire chunk of data to be read off the drive in parallel, giving this type of arrangement huge bandwidth. When one sector on one of the disks fails, however, the corresponding sector on every other disk is rendered useless because part of the data is now corrupted. RAID 0 does not implement error checking so any error is unrecoverable. More disks in the drive means higher bandwidth, but greater risk of data loss.
* RAID 1: Mirrored Set (2 disks minimum) without parity. Provides fault tolerance from disk errors and single disk failure. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, very small performance reduction when writing. Array continues to operate so long as at least one drive is functioning.
* RAID 3 and RAID 4: Striped Set (3 disk minimum) with Dedicated Parity, the parity bits represent a memory location each, they have a value of 0 or 1, whether the given memory location they represent, is empty or full, thus enhancing the speed of read and write. This mechanism provides an improved performance and fault tolerance similar to RAID 5, but with a dedicated parity disk rather than rotated parity stripes. The single disk is a bottle-neck for writing since every write requires updating the parity data. One minor benefit is the dedicated parity disk allows the parity drive to fail and operation will continue without parity or performance penalty.
* RAID 5: Striped Set (3 disk minimum) with Distributed Parity. Distributed parity requires all but one drive 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.
* RAID 6: Striped Set (4 disk minimum) with Dual Distributed Parity. Provides fault tolerance from two drive failures; array continues to operate with up to two failed drives. This makes larger RAID groups more practical. This is becoming a popular choice for SATA drives as they approach 1 Terabyte in size. This is because the single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt. The larger the drive, the longer the rebuild will take. With dual parity, it gives the array time to rebuild onto a large drive with the ability to sustain another drive failure.

http://en.wikipedia.org/wiki/RAID
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post #7 of 12
iamme, with 3 drives, RAID 5 uses 2 drives WORTH OF STORAGE for data, and 1 drive WORTH OF STORAGE for parity. Yes, the parity is spread out through all the drives.
post #8 of 12
Thread Starter 
Quote:
Originally Posted by demoship View Post
I'd be very cautious about using software RAID 5. I did speak w/ a systems admin a while back, and he said that using software raid w/ more advanced raid arrays can be very dicey. Using software RAID 5 can give you a false sense of security. When a drive actually fails, you might end up losing all your data anyway, even though it was "supposed" to have redundancy.
Well, what is an advanced Raid 5? Mine isn't going to be holding an OS (that's on a separate drive), nor is it going to be heavily strained.

Quote:
Originally Posted by demoship View Post
RAID 5 is also EXTREMELY slow when writing unless you have a dedicated card to it (a good one). It doesn't matter if your CPU is idle, it's just not efficient enough to do the specialized work needed for RAID. (plus every bit you're writing has to go to more places for processing, which takes more time).

If you don't want to shell out the money for real RAID 5, I advise that you use RAID 1 or no RAID.
Well, what is extremely slow? This is for backing up media, so the write speed isn't huge unless it's like <1mbps, which I would doubt. The idea is that I will use it for backing up my files and storing them, not for heavy constant writing (as if I were say, video editing).

I'll be using Windows Server 2003 and it's built in Raid-5 capabilities. Are you saying that it will be unreliable? Because that is what I care about, not the performance (Unless it's beyond dreadful as the example I mentioned.).
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post #9 of 12
Quote:
Originally Posted by demoship View Post
iamme, with 3 drives, RAID 5 uses 2 drives WORTH OF STORAGE for data, and 1 drive WORTH OF STORAGE for parity. Yes, the parity is spread out through all the drives.
I know thats what i said!
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post #10 of 12
Yes, it may be unreliable. Software RAID 5 may fail if 1 drive fails, whereas real hardware RAID 5 very very very rarely fails if 1 drive fails.
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