Web Hosting Talk : Web Hosting Main Forums : Web Hosting : RAID 5 Vs RAID 1, what is the difference?

[blob2]

5 x 146GB Hard Drvies in RAID 5 = 730GB? How is this configuration redundant?

2 x 146GB Hard Drives in RAID 1 = 146GB

I know this is a stupid question, I understand RAID 1 mirrors content but how does RAID 5 work and what is the total drive space?

[Loon]

I was once wondering the same, till i found this cool guide http://www.acnc.com/raid.html

[Glanhosts]

The biggest difference is the in raid5 lost data can be build back using raid5 incase of hddcrash and that is not possilbe inraid1

Good Luck

[cartika-andrew]

RAID5 stripes data across the drives to create redundancy. If a drive is lost, a new drive can be added, and the data will be re-striped across that drive (usually on the fly - especially with Hot Swap capable machines) utilizing an algorithm to formulate the missing data. The advantage of Raid 5, especially across 5 drives, is that you can lose more then 1 drive and still be up and running..

However, RAID5 is expensive -and the total usable drive space would less then 5*146 or whatever - usable drive space would really depend on the way the RAID array was setup and how the specific RAID controller operates - but, a good rule of thumb is

3 drives in RAID5 - 2/3 usable drive space
5 drives in RAID5 - 3/5 usable drive space

I havent looked at RAID5 in awhile, so - someone please correct me if I got this wrong...

[hiryuu]

Yeah, it's wrong. RAID5 can not take a multiple drive failure. (RAID6 can, but that's rare, expensive, and I haven't looked into it much.) RAID5 capacity is that of n - 1 drives. In this example, 5 x 146 GB would yield 584 GB of usable space.

Glanhosts: Similarly, you are thinking of RAID0 (striping), but that gives you the full capacity of all drives. RAID1 can rebuild failed drives, but you have the capacity of n/2 drives, which is no small sacrifice.

[cartika-andrew]

Yup, youre absolutely correct...

Did some research after reading your post

http://www.pcguide.com/ref/hdd/perf/...eLevel6-c.html

RAID5 & RAID6 are often thought of and referred to as the same thing, just a different configuration..

Depending how you set up RAID5, it can tolerate the loss of 2 drives - however, this is sometimes referred to as RAID6 - but, most organizations do not utilize this terminology...

Also - RAID0 typically is not referred to as striping, but, rather mirroring..

Thanks for pointing this out though, I wasnt really aware that a "RAID6" term had been coined in this capacity - as the organizations Ive been involved with always referred to this as RAID5

[BigBison]

RAID5 is I believe slower than RAID1 as it stripes rather than mirrors. I lost two drives in RAID5 one day, but had a hot standby configured on my DPT adapter and was able to swap out the other two.

[cartika-andrew]

Originally posted by BigBison
I lost two drives in RAID5 one day, but had a hot standby configured on my DPT adapter and was able to swap out the other two.

Yeah - you hit it right on there - thats the BIG difference and advantage -

[BigBison]

See, you need 3 drives to go RAID5. When the first drive fails, RAID5 brings the standby (provided you have that 4th drive) online and writes to it. The odds are slim, but now if a second drive fails you're only down to 2, which will still operate - with a massive performance hit because every read must come from the stripe set algorithm, instead of the data profile - hope that isn't too confusing. You get a further performance hit after you hot-swap the first failed drive as it rebuilds. Hot-swapping the second failed drive has no performance hit - the array is already back to 3 drives, you're just configuring the freshest drive as the new hot-standby for the array.

The larger the cache (I always prefer 16MB ECC RAM, minimum on RAID5) the less noticeable the performance hit.

The purpose of mirroring is not redundancy so much as performance. Picture a system with a single HD - can you read and write simultaneously? No. Shame, because I would bet my last nickel your OS multitasks, i.e. is capable of reading and writing at the same time. This is why you mirror - now, you're multitasking your filesystem access. This is preferred for the workstation to boost performance, but in a DC server you want robustness as well so RAID5, with as much memory as you can afford to improve performance.

When mirroring, the adapter is simply writing everything twice - once on the available drive, the second time at the system's convenience. When striping, the adapter is running an algorithm to propagate a stripe set across at least 3 drives, as well as writing the actual data to each drive.

[blob2]

So what would solution would be recommended?

[bilalk]

RAID 0 is striping.
This involves writing some of your data to one drive, and some of your data to another drive.
It is for added performance *only*, and provides no data protection whatsoever!

RAID 1 is mirroring.
All data is written to two drives, and is also read from both drives.
This gives you added performance on reads, but slows down your writes as they have to be made to both disks.

Those two types are fairly simple to understand.
They can also usually be done with a software RAID solution without taxing your CPU too much.

RAID 5 uses XOR for maximum storage space with decent reliability.
You need at least 3 disks to use it, and you end up with (n - 1) drives of storage space.
IE. If you have 5 drives in a RAID 5 array, you have 4 drives worth of space.
It gives you good read and write performance, but software-based solutions can slow down your processor and good hardware-based cards are VERY expensive.

Basically, you choose your RAID solution based on your needs.
If you're not worried about losing data and simply want the best performance you can get for a low price, go for RAID 0.
For a low-priced solution to data safety without worrying too much about performance, choose RAID 1.
If money is not an issue, get a good RAID 5 card and additional drives for both data safety and decent performance.

[KDAWebServices]

If money isn't an issue, then I'd actually say go with RAID10 (Sometimes called RAID 1+) rather than RAID5. RAID5 suffers on writes, even with a good card, especially if it's 100s of writes per second of small files (For example, mail servers) - or at least in our experience it does.

[rey]

I second KDAWebServices.

[BigBison]

Originally posted by BigBison
When mirroring, the adapter is simply writing everything twice - once on the available drive, the second time at the system's convenience. When striping in RAID5, the adapter is running an algorithm to propagate a stripe set across at least 3 drives, as well as writing the actual data to each drive.

bold = edit

Originally posted by blob2
...what is the total drive space?

I've always used RAID1+0 or RAID1 on workstations, because I want the performance.

I've always used RAID5 on servers, as I described above - on the off chance you suffer simultaneous drive failure, I'd prefer latency to go up rather than the system going down. Also, you get more efficient use of HD space when striping, as opposed to mirroring where you're doubling the space needed outright. Thus, adding more drives to RAID5 or adding another RAID5 array is more cost-effective in a server. Total drive space is dependent on your total drives and specific setup - you can configure four active drives with no standby as well, depends on your card, but the more drives you add the more efficient the space utilization in the array.