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What Kind Of Software RAID Are You Running?
ErikZ asks: "Lately, I'm having issues with my RAID. Specifically, closed source drivers for my RAID card that only support Red Hat 9. So I've decided to Ebay the card, and try to figure out how to turn 4 SATA drives into a software driven RAID 5 setup. Yes, I know I'll lose all the data, and I'm not worried about it. Finding a 4 port (or more) SATA controller card, that's well supported under Linux, has been difficult. Everyone wants to slap on their own RAID chip and charge you another 100$ for the pleasure. Where can a guy get a highly recommended, well supported, 4 port SATA card for Linux? The Rocket 1540 cards have vanished off the face of the earth. There are a few motherboards out there that have 4+ SATA connectors on them, but they also add RAID and some other cutting edge features that aren't well supported under Linux.
So, I thought I'd try another route and ask Slashdot: What are you using for your Linux software RAID needs? What do you suggest?"
I took some notes when I was setting up my home 4x250GB RAID 5 server. I found there to be three categories of RAID solutions. Might be helpful for you in deciding. Copied them below and added a few extra comments.
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It really depends on what you are using the server for. I ended up going for the pure Software RAID option. Its for home and I'm a cheap. If you're not cheap or it is for a work server, I'd stick with the pure Hardware solutions.
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Hardware RAID:
The expensive Adaptec, 3ware, etc SCSI cards found in most servers.
Pro - Offloads XOR calculations from the CPU to internal processor.
Pro - No manual intervention required in case of a raid failure.
Con - Expensive.
Con - Third Party and/or closed source drivers often required.
Semi-Hardware RAID:
These are typically the SATA RAID controllers that are built into motherboards but includes cheapo add-on cards. Generally if less than 150 bucks, not full hardware RAID. I believe all of the RocketRAID cards fall in this section.
Pro - No manual intervention in case of a disk failure.
Pro - Cheap.
Con - Minimal or No CPU offloading.
Con - Third Party and/or closed source drivers often required.
Software RAID:
Use Linux and plain old SATA/PATA controllers to handle all of your RAID needs.
Pro - Very cheap.
Pro - No worry about driver incompatiblity or closed source drivers.
Con - No CPU offloading. You essentially trade CPU power for Disk speed/redundancy... and its a significant trade.
Con - Manual intervetion required in case of disk failure.
Con - PATA Only. Must be one drive per channel! NO SLAVES! Apparently data loss can occur on both drives in the chain if one goes bad. http://www.tldp.org/HOWTO/Software-RAID-HOWTO-4.h
Performance is also hurt in a Master/Slave combo.
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The general consensus among linux kernel engineers and software RAID users is:
1. As long as the onboard SATA chip is well supported on your linux kernel, use the onboard chip.
2. Don't worry about the "hardware RAID" built into the motherboard. You don't have to use it. In fact, most people bypass it.
3. Use the non-BIOS SATA driver for your motherboard. Some motherboards have two different chips. Mine (an Epox 8RDA+Pro nForce Ultra2/400) uses both the common Silicon Image SIL3114 which supports 4 SATA drives and an additional 2 SATA drives provided by the onboard nForce 2 Ultra Gigabit MCP chipset. Quite nice for RAID and I still have normal PATA IDE drives 0 - 3.
4. Quite often the SATA RAID hardware only supports RAID 0,1 and 10 (or 01 depending). If you're looking for RAID 5 then you'll have to buy a more expensive outboard solution. The problem with outboard solutions are that they will eat into your PCI bandwidth. If you will be using PCI-X then you will probably also be paying significantly more for your outboard solution. Most people have a ton of CPU lying around, so handing off the I/O doesn't really buy you that much.
5. When it comes down to it you might as well just use software RAID because you have more control over it. You can use the onboard SATA controllers which allow you to take advantage of the increased on-motherboard bandwidth as well as having a significantly less expensive solution.
6. Another advantage to using Linux software RAID is that you don't have to learn a new RAID management system everytime you upgrade your machine and controller. You can also connect to your machine remotely and manage your raid system through a firewall. Sometimes you can do that with your hardware RAID system and sometimes you need to manage it from the BIOS itself.
7. Once you get comfortable with software RAID you can experiment with mixing and matching various I/O systems underneath it. One of the things I'd like to play with would be using software RAID with Firewire 800 external drives in a pseudo-SCSI arrangement.
8. The LVM2 system doesn't need software RAID, but it works very nicely with it none-the-less and gives you snapshot support etc.
9. Personally, I'm going for RAID 10 (striped mirroring) because drives have gotten very inexpensive and I don't mind burning a few more to get higher I/O rates. Remember, if you go with a mixture of RAID 0 and 1 then you want a striping over mirroring -- that way if you have a single drive failure the array keeps going.
Have fun and don't use RAID instead of backups. Backups save the stuff that you deleted intentionally but need to recover.