RAID Vs. JBOD Vs. Standard HDDs

Ravengbc writes "I am in the process of planning and buying some hardware to build a media center/media server. While there are still quite a few things on it that I haven't decided on, such as motherboard/processor, and windows XP vs. Linux, right now my debate is about storage. I'm wanting to have as much storage as possible, but redundancy seems to be important too." Read on for this reader's questions about the tradeoffs among straight HDDs, RAID 5, and JBOD.

At first I was thinking about just putting in a bunch HDDs. Then I started thinking about doing a RAID array, looking at RAID 5. However, some of the stuff I was initially told about RAID 5, I am now learning is not true. Some of the limitations I'm learning about: RAID 5 drives are limited to the size of the smallest drive in the array. And the way things are looking, even if I gradually replace all of the drives with larger ones, the array will still read the original size. For example, say I have 3x500gb drives in RAID 5 and over time replace all of them with 1TB drives. Instead of reading one big 3tb drive, it will still read 1.5tb. Is this true? I also considered using JBOD simply because I can use different size HDDs and have them all appear to be one large one, but there is no redundancy with this, which has me leaning away from it. If y'all were building a system for this purpose, how many drives and what size drives would you use and would you do some form of RAID, or what?

Comments

Re:Get what you need for *NOW* not for later

[QuesarVII]

By the way RAID 5 is a pain in the ass unless you have physical hotswap capability, which I highly doubt.

With recent kernels, you can hotswap drives on nvidia sata controllers (common onboard). I believe several other chipsets had support for this added in recent kernels too. Then you can swap drives live and rebuild as needed.

One more important note - if you're using more than about 8 drives (I personally recommend 6), I would use raid 6 instead of 5. You often get read errors from one of your "good" drives during a rebuild after a single drive failure. Having a 2nd parity drive (that's what raid 6 gives you) solves this problem.

You can mix raid drive sizes, with planning.

[Kaenneth]

You can put RAID 5 on varying size disks.

I had 4 300GB drives, and 2 200GB drives.

I broke them up into 100GB partitions, and layed out the RAID arrays:

A1 = [D1P1 D2P1 D3P1 D5P1]
A2 = [D1P2 D2P2 D4P1 D6P1]
A3 = [D1P3 D3P2 D4P2 D5P2]
A4 = [D2P3 D3P3 D4P3 D6P1]

Then I concatenated the arrays together, giving a little less than 1.2 TB of space from 1.6 TB of drives; if I had just RAID'd the 4 300 gig drives, and mirrored the 200's I would have only had 1.1 TB available, and the drive accesses would be imbalanced.

I could also grow the array, since it was built as concatenated, so later when I got 4 400GB drives I raided them then tacked them on for 2.4 TB total.

Re:Linux, RAID 5, md

[ptbarnett]

I did exactly this for a new server recently. The only thing I would add is to use RAID 6 instead of RAID 5. That way, you can tolerate 2 drive failures, giving you time to reconstruct the array after the first one fails.

I have 6 320 GB disks. The /boot partition is RAID 1, mirrored across all 6 (yes, 6) devices, and grub is configured so that I can boot from any one of them. The rest of the partitions are RAID 6, with identical allocations on each disk.

There's a RAID HOWTO for Linux: it tells you everything you need to know about setting it up.

Infrant X-RAID is the solution

[JoeShmoe]

Infrant (wow, just checked their website and it looks like they were bought by NetGear) created their own version of RAID that specifically addresses the issue of capacity and expansion. It's a nice transitional blend from RAID-1 to RAID-5 and does offer the ability to increase the total capacity (albeit with a lot of drive swapping).

Buy an Infrant RAID with the two biggest drives you can afford. Let's say two 750GB drives or whatever's on sale that week. It starts out acting as RAID-1 with the drives mirroring. So you have 750GB of "safe" storage. Now you add another 750GB drive. Okay, now you have 1500GB of storage with one of the drives acting as parity drive (RAID-5). Add a fourth drive and how you have 2250GB of "safe" storage. Now you come back and just replace one of the original 750GB drives witha 1TB drive. Do you get extra capacity? No...not initially. But the drive is fully formatted and integrated as X-RAID. What this means is that eventually after you have piecemeal or onesie-twosie upgraded all four drives, suddenly the X-RAID resizes itself to match the capacity of the new drives with no transfer or downtime. So in theory if you wanted to upgrade your RAID, buy four 1TB drives, swap them out one at a time (letting each one rebuilt the array) and then at the end you'll have 3TB RAID isntead of the old 2250GB RAID and all the data intact.

http://www.infrant.com/products/products_details.p hp?name=About%20X-RAID

I have three ReadyNAS units and love them to death. They are a little fussy about drive temperatures (I guess that's a good things but, I may get like 40 emails during the course of the day about it and it's not like I'll drive home from work to turn up the A/C in my house). My only sadness is that Infrant doesn't have a higher capacity unit than four drives (oh please oh please, eight drives with a RAID-6 type protective hotspare in one nice rack-mountable unit would be my ultimate dream).

-JoeShmoe
.

RAID 5 is damned easy.

[SanityInAnarchy]

This is what you do: buy 2 drives exactly the same size and mirror them. End of story.

Until another few years go by and you want to buy more storage. Then you're basically stuck with doubling it, clumsily -- or migrating away and essentially throwing out the old drives.

RAID 5 is better in the short run. Even with a three disc array, you're getting more storage for your money, and you can always restripe it onto a fourth disc.

(If you need more than 500 GB I would highly suggest encoding your porn into a different format than MPEG2)

It's not all porn, and some of it is high def, in h.264. And I don't even edit videos, I just watch 'em.

With computers, the stupidest thing you can do is spend extra money to prepare for your needs for tomorrow.

That is true. However, I would fill a terabyte easily, and right now, I'm guessing it's cheaper to buy three 500 gig drives than two 1 tb drives.

By the way RAID 5 is a pain in the ass unless you have physical hotswap capability, which I highly doubt.

You highly doubt he's got SATA?

The one thing I will say is, either have another disk (even a USB thumb drive) to boot off of, or do some sort of RAID1 across them. You almost certainly want software RAID on Linux, and you don't want to try to teach a BIOS to boot off of your array.