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Raid 5 Questions

1844 Views 7 Replies 6 Participants Last post by  ComGuards
I am planning on building a 3 1.5TB HDD Raid 5 setup. I was thinking about putting it in my gaming machine, or maybe building a small server for it. It will be to hold my HD video so I can stream them to my PS3 and other things, and if I build a small server for it I will probably host a Voice server and maybe a game server from that.

I had a few questions about Raid 5.

If I wanted to add a 4th or even 5th rive at a later point, can I just add it to the raid with losing the data.

If I put it in my gaming machine, will the Raid 5 survive a mobo swap when I upgrade the computer. As I often upgrade hardware quite often, and plan on upgraded the processor/mobo/ram in the next year.

Will a raid 5 survive a OS switch/reinstall, as I do that often on my gaming machine.

If I plan on building a small server and have the raid 5 with voice and game server on it do I need anything more then a single, or should a single core be fine?

If I set up the Raid 5 in my gaming machine will it have any performance penalty on my machine?

If I do put it in my gaming machine, and upgrade CPU/MOBO later can I just take this MOBO and CPU and turn it into a server for my RAID 5? Just take the MOBO and CPU out and put it in another case and put a new OS on it.
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Onboard RAID5 is not expandable, but you can do so with most add-in cards.

Onboard RAID5 is very slow to write - decent hardware cards remove this bottleneck.

If you upgrade your mobo to one with a similar chipset you can move the array - but different controllers will mean your array is not transferrable. An add-in card can be moved between systems without issue.

If you don't install your OS to the RAID5 then you can reinstall as often as you like. You should not run RAID5 for your OS drive anyway.

If you intend to use software RAID5, a dual core cpu might be a good cpu - this gives a core for the TCP/IP overhead and another to do the parity calcs. If you buy a hardware card then a single core is fine. If you run software RAID (or onboard RAID5) there is a slight cpu hit - whether or not this actuall affects framerates is another matter though - providing you don't use the array for your games then it won't make any difference.

I would recommend either a linux software RAID5 or a PERC 5 hardware based RAID5 array, running in a dedicated server. Don't use onboard RAID5. You will not need any particularly fast cpu - get an E3200 or a Sempron.

I think that has answered most of your questions - if you want to know any more detail let me know...
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I will definitely have a separate HDD for the OS and anything installed on the computer. I don't believe in partitioning or even using 1 HDD for multiple things. Even in my gaming machine I run a WD 150GB Raptor for the OS and small installed programs. And a separate 320 GB 16mg cache Seagate for all my installed games.

The only writing to the raid that would occur is when I put a new movie on it, which will be from 8-12gb in size. After that it would only be read from.

If I wanted a expandable raid what would my options be?

I planned on either using the on-board raid on my machine, or a on-board raid in a server I built to run it. As this seemed the most cost effective way to do things. I also planned on running it on a Windows based OS just for sheer ease of use and compatibility. What are the benefits of doing it another way?

Also what would be a good PCI card to go with if I decided to use a PCI card the setup.
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Originally Posted by the_beast View Post
If you don't install your OS to the RAID5 then you can reinstall as often as you like. You should not run RAID5 for your OS drive anyway.
Why shouldn't you run an OS on a RAID 5?

I do so now on my LSI 8303ELP without issue... The only area that can be seen as a con is the write performance hit that a RAID 5 lands, but having a dedicated card makes that a non-issue. At the slowest my write speeds are the same as a single drive configuration, but more often it is about 2x as fast as a single drive.
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RAID 5 does combine all the best of other levels but it uses more memory,recovery takes longer, minimum is 3 drives,if more than one fails all data is lost and must be restored from backup.
Its just slow. I regret ever using it for my OS. But luckly my two Caviar blacks should be here wednesday!
RAID5 is a bad idea for OS because it has TERRIBLE small write performance. A hardware controller gets away from the problem of slow large writes, and can almost match the performance of comparable RAID0 arrays for large writes. The onboard cache can also help a little. But nothing can remove the problem.

When writing less than 1 stripe-width of data, the controller must READ all of the data on the disks, re-calculate the parity, then re-write the data back to the disks. This takes time, and ties up your disks with a lot of extra I/O requests. As your OS does lots of small writes continuously, it is a poor idea to run RAID5 in this manner.

There is no 'best RAID level' - each one has is uses, hence the reason why there ARE multiple levels. If there was a best option we would all be using it...

Onboard RAID5 is terrible - seriously don't use it. The speeds are poor, the array management tools and recovery options are poor. If you want RAID5 (and for many applications - such as media storage - you do want RAID5, or even RAID6 when you have a few more disks) you really have 2 options. Firstly *nix software RAID. This is lightyears ahead of Window's RAID options, and is available in pre-built distributions designed to run servers. If you want a 'Home' server look at Amahi, for more features & rock-solid reliability, but steeper learning curve, check out Ubuntu Server or similar. For purely storage options look at OpenFiler.

If you want Windows, or want to go hardware RAID5, the only budget option is the Dell PERC5/i from eBay (or the faster, RAID6-capable PERC6/i). The 5/i goes for around $100, the 6/i around $150. They are easily the best cards for the money - anything comparable is $450, and I would not recommend anything cheaper, as all cheap cards suffer from the same problems as onboard systems. Basically if your RAID card does not have a heat sink it is garbage. There are a few caveats with the PERC cards though - check out the (massive) thread in the RAID subforum.
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I echo everything that The_beast said.

It really comes down to a matter of compromise. RAID levels offer various levels of compromise. You want speed, you give up either space or redundancy. You want redundancy, you give up speed and / or space. You want space, then you give up speed.

Honestly, from a personal perspective, I wouldn't recommend RAID-5. I've worked with 4 generations of Dell and HP servers, all running hardware RAID with SCSI or SATA/SAS drive configurations, and I just remember painful experiences with those systems running RAID-5. Granted, with 74GB & 147GB SCSI drives, maximizing storage space was the main concern, so RAID-5 offered that with *limited* redundancy and a noticeable hit on performance.

These days though, with SATA drives that much cheaper, "more storage" isn't really becoming a huge issue either.

In any case, with regards to the OP. The best way of implementing what you want would be to go with a PERC5- or equivalent-class of hardware RAID controller. That way you can move it between systems. You'd want to go with PCI-e controllers. Definitely not standard PCI. Even PCI-X has little to recommend it these days.

I am personally a big fan of RAID-10. If I had it my way, I'd be running RAID-10 on all my systems. Granted, you lose half the space you put into it, but you get wicked performance out of it. And you can lose up to half the drives in the array, theoretically. Depends on which drives, of course.

Another thing to remember. If your array is going to be managed by a Windows system, i.e. Windows Disk Management, you're limited to 2TB partitions unless you upgrade the "disk" to a GPT disk, instead of a standard MBR disk. Won't go into the details of each, enough Google articles hit on it. But something else for you to keep in mind.

Also, if I remember correctly, the PERC5i actually also has the same limitation. The "virtual disk" you create is also limited to 2TB in size. So if you put 3x 1.5TB in RAID5, you end up with a virtual disk of 3TB. I'm fairly certain that initialization would fail. That was a limitation of the hardware. PERC6 doesn't have that limitation. In such a case, you would just have to build two separate virtual disks - 1 of 1TB, and 1 of 2TB.

Heh, if you can afford 3x1.5TB, just bite the bullet and get a 4th drive and go RAID-10.
With a PERC5 or PERC6 card, those come with the ability to handle 8 drives. If you need more space, just add another 4 drives and create another array on the same controller.
You get the same amount of usable space (3TB) but near RAID-0 write speeds. And it can theoretically tolerate a 2-disk failure

If you want any kind of alternatives... hrm.... I also run an Adaptec 2405 PCI-e RAID card with 4x320GB WD-RE drives in RAID-10.

Then again, the question is, do you really need any kind of redundancy on your movies?
Hard-to-find stuff, maybe, but stuff that can be re-downloaded, not so big of a deal

Anyways. Stay away from software RAID-5, you'll just end up hurting yourself. :swearing:
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