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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?
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?
Nothing can possibly go wrong. Especially if you use, like, 10 disks.
Wikipedia has a very informative article regarding RAID and the various levels, in fact here it is. http://en.wikipedia.org/wiki/RAID
Chicken fried butter sticks? Do
That said, RAID is not a replacement for proper backup. RAID is just a first line of defense to avoid downtime.
"No one likes working in a hamster wheel, and your shop smells of cedar shavings from here." - TaleSpinner
You can just download them again, right?
This issue is a bit more complicated than you think.
This is what you do: buy 2 drives exactly the same size and mirror them. End of story. If you're worried about a blown raid controller, then buy another hard drive and stick that on another computer and run a weekly cron job to copy everything. Right now you can get 500 GB hard drive for about $150. Get two of them and mirror them. (If you need more than 500 GB I would highly suggest encoding your porn into a different format than MPEG2) By the time you run out of space, you will be able to get 1 TB drives for about $150. Migrate over to the 2 1 TB hard drives. Repeat every few years.
With computers, the stupidest thing you can do is spend extra money to prepare for your needs for tomorrow. Buy for what you need now, and by the time you outgrow it, things will be cheaper, faster and larger.
By the way RAID 5 is a pain in the ass unless you have physical hotswap capability, which I highly doubt.
Media Server: n. A euphamism for digital porn storage.
With Linux you can create a RAID5 md device, say /dev/md0, then run LVM on top of that (pvcreate /dev/md0 ; vgcreate MyVgName /dev/md0) and use that to carve out your storage. The key here is to create partitions on each drive, eg filling up the entire disk, and create your raid5 with those.
/dev/md1. So now you have /dev/md0, pointing to the first 500GB of each disk, and /dev/md1, pointing to the 2nd 500GB of each disk.
/dev/md1 and graft it onto your LVM volume group. (pvcreate /dev/md1 ; vgextend MyVgName /dev/md1). Now your LVM VG just doubled in size, and you can use all that new space. Whatever you do though, do NOT create any "striped" logical volumes (the "-i2" option to lvcreate; LVM's Poor Man's RAID0, basically) because you will suffer terrible performance, since you'll be striping across different volumes on the same physical spindles (a big no-no for any striped configuration). But if you use the extra space by creating new filesystems or growing existing ones, you shouldn't see any trouble.
If you buy 1TB drives further down the road, here's what you do- With each disk, create a partition identical in size to the partitions on the smaller disks, then allocate the rest of the space to a second partition.
Join the first partition of the disk to the existing RAID set. Let it rebuild. Swap the next drive, etc. etc. Then once you've done this switcharoo to all the drives, create another raid set using the 2nd partition on your new disks--call it
Take that
Just be sure that any replacement drives you have to buy... you must partition them out similarly. I'd recommend pulling back on the partition sizes a bit, maybe 5%, to account for any size differences between the drives you bought right now and some replacement drives you may purchase later on which might be slightly lower in capacity (different drive manufacturers often have differing exact capacities).
the real at&t mix
Go RAID5. RAID5 = Hardware failure resilience + maximum storage.
Go Linux. The Linux MD driver allows you to control how you RAID- over disks or partitions. there are advantages. We will discuss.
First, don't get suckered into a hardware RAID card. They are *NOT* really a hardware card- they rely on a software driver to do calculations on your CPU for RAID5 ops. Software RAID is JUST AS FAST. Unless you blow the big bucks for a card with a real dedicated ASIC to do the work, you're fooling yourself.
Now, you want to go Linux. By using the md driver, you can stripe over PARTITIONS, and not the whole disk. By doing this, you can get MAXIMUM storage capacity out of your disks, even in upgrades.
Say you have 3 500GB disks. You create a 1TB array, with 1 disk as parity. On each of these disks is a single partition, each the size of the drive. Now, you want to upgrade? SURE! Add 3 more disks. Create three partitions of EQUAL size to the original, and tack it on to the first array. Then, with the additional space, you can create a WHOLE NEW array, and now you have two seperate RAID5's, each redundant, each fully using your space.
Another advantage with MD is flexibility. In my setup, I use 5x 250 drives right now. On each is a 245GB partition, and a 5GB partition. I use RAID1 over the 5's, and RAID5 over the rest. Why? Because each drive is now independently bootable! Plus, I can run the array off two disks, upgrade the file system on the other 3, and if there's a problem, I can always revert to the original file system. So much flexibility, it's not even funny.
I recommend using plain old SATA, in conjunction with SATA drives, and just stick with the MD device. For increased performance, watch your motherboard selection. You could grab a server oriented board, with dedicated PCI buses for slots, and split the drives over the cards. Or, you can get a multiproc rig going, and assign processor affinity to the IRQ's- one card calls proc 1 for interrupts, the other card calls proc 0. If you have multiple buses, then performance is maximized.
The last benefit? Portability. If your hardware suffers a failure, then your software RAID can move to any other system. Using ANY hardware RAID setup will require you to use the EXACT same card no matter what to recover data. Even the firmware will have to stay stable or else your data can be kissed goodbye.
Windows? Forget about it.
Good luck!
I really can't believe this made the front page. The questions are badly written, and the question itself could have been answered with some basic Internet research. RAID isn't an esoteric topic anymore, folks!
This place has really gone downhill. I thought Firehose was supposed to stop stuff like this, not increase it!
Anyways, just to be slightly on topic: there's no one answer to this question. It depends on your budget, your motherboard, your OS, and, most importantly, your actual redundancy needs. This kind of thing is addressed by large articles/essays, not brief comments.
Plausible conjecture should not be misrepresented as proof positive.
Actually, the failed hard disk will personnaly walk to you.
No sig for now.
Raid 0 won't protect you, man!
The advantages of RAID 0 versus RAID 1 versus RAID 5 have already been covered in detail, here, and in many books and websites.
However, allow me to address the issue of how they relate to a media center:
Firstly, when you say "media center/media server", do you mean "I just want to build myself a kickass Tivo?", or do you mean "I want to serve video for everyone in my frat house, simultaneously?"
If the former, consider that Tivos ship with 5500 RPM drives for several reasons:
1) They're cheaper than faster drives
2) They run cooler than faster drives
3) They run quieter than faster drives
4) They use less power than faster drives
5) They're more than fast enough for streaming a single video to your TV while recording another
Long story short, if you're just building a "free" Tivo with a kickass drive array, performance is *not* an issue. Keep in mind that if you're building a set-top box of sorts, the low heat and low noise features are *very* big benefits. You probably want RAID 5, and/or JBOD.
If, however, you're planning on serving video to more than a handful of stations simultaneously, you may need to consider performance. This is a vote for RAID 0 and/or RAID 10.
Now, the second axis: How important to you is this data? Really?
I've got over 300 gigs of drive space on my Tivo. Most of it is the last two weeks of television reruns (Scrubs, 6 copies of last Thursday's Daily Show, etc.), movies I recorded but won't watch, etc. There are about 10 gigs (3%) of video on there that's been saved for a few months, and frankly, I couldn't tell you a single thing on there that I'd miss if my drives went belly up tomorrow. So: do you *really* need to save all those Seinfeld reruns on a highly-redundant storage array? How *much* of the stuff on the server do you really need to keep?
Assuming it's less than 50% (in the Tivo scenario, it probably is), consider using JBOD for most of your storage, and maintaining a single backup drive, or small backup drive array. Or just backing up the good stuff to DVD.
In summary: If you're just building a Tivo, you probably don't really need the performance, or redundancy that RAID offers.
Seriously though, how hard is it to type "RAID" into Google?
Not hard at all...
This guy's the limit!
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).
p hp?name=About%20X-RAID
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.
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
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-- I wonder which will go down in history as the bigger failure: the War on Drugs or the War on Filesharing
3ware made some pretty good cards.
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.
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.
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.
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.
Don't thank God, thank a doctor!
That's why you use hardware RAID. A good card will allow you to swap out drives and rebuild, or add new drives to the array, without ever needing to unmount the anything.
But the minute or so of uptime you get by not having to power down the computer is more than made up when the controller chip on your beautiful RAID controller sizzles. Using Linux software RAID lets you plug the drive(s) into another computer of a completely different chipset, boot up, and continue operations as though nothing had ever gone wrong. IMHO, this is far preferable to the effective lock-in presented to you by hardware controllers.
For me, it's all 100% software RAID 1.
I have no problem with your religion until you decide it's reason to deprive others of the truth.
Eh?
LVM and RAID are orthogonal solutions, and don't do the same thing. LVM will let you make a single larger partition out of a number of real partitions, and before anyone says that's the same as RAID0, I should point out that RAID0 is not a real RAID level (as it has no redundancy). The circumstances for failure for LVM and RAID0 (JBOD too) are basically the same - if one part fails, you will quite possibly lose the whole lot.
As for hardware RAID, I would not necessarily recommend that either, as it moves the single point of failure without resolving the problem. Replacing a broken controller with something compatible some years down the road can prove impossible, especially with onboard controllers. There's also the fact that a number of RAID controller cards are buggy and others do most os the work in software drivers anyway! Performance is also no longer a reason to use a pure hardware RAID solution, especially now that multi-core machines are available cheaply.
Hot-swap is still someting that requires a good hardware solution, but that's about it. Good (and well supported) RAID products cost good money too, and for most of us it's just not worth doing - better to use software RAID, buy more RAM, and pocket the rest.
-- Steve
I think the comment about CPU performance was more about the fact that with faster CPUs, the speed benefit of a hardware raid solution isn't as useful. I checked the raid6 personality on my 1Ghz celeron, and it reports that it can calculate RAID parity at a throughput of 985MB/s, using the SSE parity calculation routines. That's more than you do any useful file serving with (it has to go out on the network, and that'll saturate gigabit ethernet). The I/O performance advantage of a hardware controller doesn't seem too useful here. I'm also not sure why software raid can't benefit from the multi-channel read performance available with raid 1, 5, etc.... Why can't the software issue a read command to two drives simultaneously? The comment about a buggy raid controller obviously wasn't talking about a new controller, but one that has eventually failed. I imagine they generally become obeselete before they fail, but even so, a failed controller really sucks if you need to figure out how to get the raid stack running on another controller. I had a quick look into it at one stage, and it broke my brain trying to work out what controllers worked with what....