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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?"
It's been running my raid for a couple of years...I think. I cannot honestly remember when I first configured it.
There are some things to be aware of. If you want to mount / as a raid, it can be tricky. The initrd needs to be properly configured, or the drivers must be built into the kernel.
Sometimes, the raids don't shut-down completely. I've never been able to completely solve this problem. Most of the time it's OK, but some machines have trouble. The most common culprit has been NFS.
GRUB & LILO need some special care when you plan to mount root from a raid mirror. It does work and it has all of the appropriate relibility features.
If it's not included, the full package is mdadm. There are a number of tutorials on the web for it. It's easy to set up, and easy to run (just ignore it). I don't remember if it can work with hot swapping (I don't need that yet), but I'm using it on several systems. I set it up, and I haven't had to worry about it since.
When I first got it, I stuffed a lot on the raid drive, disabled it, wiped out one disk, and re-activated the raid. It rebuilt it and worked fine.
I asked this question on a Debian (or Debian based) user list at a time when a lot of experienced admins were around, and overall the feeling was that there was no need to go hardware and the software raid would do the job.
I've considered setting up software raid on my Linux server, but I haven't found any doc yet about what happens in the event of an unexpected crash or poweroff part way through writing a RAID-5 stripe.
Suppose I have 4+1 disks in a RAID-5 configuration, and during a write to a stripe of the disk only two of the disks are written to before the system crashes. This leaves me with 2 disks with new content, 2 disks with old content, and a useless parity.
I found a page at RedHat that indicates that as of 2001 there was no multistage commit. Has anything changed since then? Do the Linux MD tools address this?
You can't just ask what RAID solution to use; you need to specify your optimization criteria. For example, if you are going for inexpensive, high capacity, and good redundancy but don't care about hotswapping, you can go with a reasonably cheap PATA or SATA hardware RAID. If you are willing to skip high capacity you can get an external SCSI SCA rack, an inexpensive SCSI card, and a bunch of small SCSI SCA drives. If you want high capacity and high performance without too much cost, get a bunch of large PATA drives, a similar number of FireWire or USB 2.0 (or both) enclosures, as many FireWire or USB 2.0 interface cards as necessary, and do it that way (which also gives you hotswap). If you are willing to spend lots and lots of money, get a Network Appliance server.
Personally, I'm not worried about performance beyond being able to play full-motion video. I have a PPC 604 180MHz from 1997 with a SCSI card and a RAID rack. 8x18GB at RAID 5 gives me 118GB or so of redundant storage, and I serve it over NFS to my other machines. Just for kicks, I have it going through a cryptoloop, too (LVM on cryptoloop on Linux RAID5 on SCSI). The initial cost was low (the drives were $15 each, the rack was around $100 on eBay, the trays were given to me, the SCSI card was under $40 on eBay, the 100Mbit ethernet card was about $20, and the computer had been a doorstop until I put Linux on it). The ongoing (electricity and cooling) costs are a little high (they are 10K drives), but that's life. I can play an MPEG or AVI from two machines on the network at once without hiccups, so I'm happy.
If I were going to build a RAID server today, I'd probably buy a Mac Mini, four large PATA drives, and four FireWire enclosures. Assuming 160GB drives, I'd have 320GB of RAID5 storage available over NFS (with a spare drive to swap in) for an investment of under $1200, and I can vary that cost with the size and number of drives. Yes, I'd be daisy-chaining FireWire, which means that each drive has only a portion of the total bandwidth. Then again, my network card will only manage 100MBit, so 3/4 of the FireWire bandwidth will be of minimal use anyway (except for reducing latency due to readahead and such, of course).
On Usenet I posted a detailed description of how I built a 2.8TB RAID storage array for under $4100.