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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?
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
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!
3ware made some pretty good cards.