Ask Slashdot: IDE Software RAID?

Edward Schlunder asks: "After setting up Software RAID on a SCSI system at work, I want to do the same at home for fun. Call me crazy, but I'm just completely geeked up about this after seeing it working. The Software RAID documentation says that each hard disk should be on a separate IDE cable and that RAID5 requires at least 3 hard drives. I want to use my two existing IDE hard drives and get the large, fast, and cheap IBM IDE ATA/66 Deskstar 22GXP hard drive to make up the third..." There's one small problem though. Hit the link for more.

"My motherboard only has two IDE ports. So, my question is, what IDE controller card can I get that satisfies the following:

• Supports Linux (obviously!)
• High speed, preferrably ATA/66 and PCI
• Lets you use multiple controllers in one system (that is, it can co-exist with the onboard IDE controller on my SuperMicro P6DBE motherboard)

Please refrain from suggesting that I should just use SCSI -- the goal here isn't absolute greatest speed and reliability, but a cheap way to teach myself more about RAID5 and provide a test system to blow things up on without causing users unnecessary grief ;-)"

Comments

controller/prebuilt system

[Anonymous Coward]

For controllers, western digital has URL's at this main link. Click on the "Solutions" link to go to suggested solutions for common problems with UDMA/66. For prebuilt IDE RAID system, try here.

Try software RAID.

[Brian Feldman]

As long as you have good IDE controllers (no huge bottlenecks), try FreeBSD's RAID/LVM system "Vinum." It would require trying an OS other than the media baby of today, but that's definitely worth it anyway.
If you _REALLY_ want to see great performance, try FreeBSD using Vinum and setting SoftUpdates on on the Vinum volume.

(Now just watch this be moderated down for being a troll, because I suggested something different...)

Misc. Topics

[Robert Bowles]

Sorry for the top-level post. Instead of several 2nd level replies, I thought I'd try to answer a bunch of questions at once...

1. Do you need 3 channels?
   No. In fact with 2D/1C (2 disks on 1 ctrlr), you will still get getter performance 1D/1C in most cases. Under general use, you're doing small-size reads distributed across the disk, so the real bottleneck is head-seek. Even with big contiguous-block reads, you'll still notice an improvement.
2. RedHat Kernels and Autodetect
   First off, RH-kernels are far from stock linux kernels. Do an 'rpm -qpl [file].src.rpm' on one of their kernel SRPM's and you'll see a bunch of (non-dist) patches. Amoung them is the raid patch
3. Promise Cards
   Support for the new Ultra/66 hasn't hit the 2.2.x tree yet(I think). Check 2.3.5+ for new Ultra/(33,66) support.
4. Raid 5 on Three Disks
   1. Doable: yes. Advisable: no.
      ( I've never tried it ) I suspect that you'd might see (marginally) better read-speeds, but you might even see degradation on write or mixed rdwr perfs ( since every write yanks two out of three heads across the platters )
   2. Two small and one big?
      This won't work for raid5, not unless you want most of the large disk unprotected. Consider instead striping (for example) hda3+hdb3==md0, and then making a raid0 or raid1 volume md0+hdc3==md1.
      Better yet, get four disks of the same size...
5. Identical Drive Myth
   For hardware raid controllers, yes, go with identical disks. This is not needed for any kind of s/w raid I've dealt with (linux, disksuite, veritas-vm, xlv). For linux s/w-raid, you should be safe making a raid5-vol by mixing two ide-partitions, a scsi-disk, a loopback off of a file and a few NBD's (so long as they are the same size).
6. Hotswap-IDE
   Scary, risky and very unwise.
   So I'm not the only one...

Software RAID

[GoRK]

To clarify the point, software RAID under Linux (any mode) does not absolutely require that each hard disk be on a seperate controller. I have had plenty of success using Software RAID on drives on the same controller. I haven't seen system performance bog down too much with this configuration either. On the newer bus-mastering Ultra33/Ultra66 controllers, CPU time for IDE access isn't really as big a problem as it used to be. So, if you're just talking three drives for a test machine, I don't know that the extra expense for a slick PCI IDE controller is going to be all that justified. Try it with your onboard controllers and then upgrade if you decide you need it.

Another question is this: Is there any support in Linux for IDE Hardware RAID controllers like the Promise FastTrack, FastSwap Pro, or SuperTrak? Obviously, Hardware IDE RAID solutions are much less expensive than traditional SCSI RAID controllers and drives and can offer comparable performance on smaller workstations or smaller workgroup servers.

~GoRK

IDE Successfully running of Linux (RedHat 6.0)

[FlameSnyper]

I'm running a software IDE raid on my box, Pentium 233 MMX, and it runs fine, if a little slow. Using the default 2 channels built into the motherboard, I have PriMaster as the boot drive and the PriSlave+SecMaster+SecSlave as the Raid5. I'm sure it would be faster with separate controllers / cables for each drive, but I was looking for redundancy, not speed.