Experiences w/ Software RAID 5 Under Linux?

MagnusDredd asks: "I am trying to build a large home drive array on the cheap. I have 8 Maxtor 250G Hard Drives that I got at Fry's Electronics for $120 apiece. I have an old 500Mhz machine that I can re-purpose to sit in the corner and serve files. I plan on running Slackware on the machine, there will be no X11, or much other than SMB, NFS, etc. I have worked with hardware arrays, but have no experience with software RAIDs. Since I am about to trust a bunch of files to this array (not only mine but I'm storing files for friends as well), I am concerned with reliability. How stable is the current RAID 5 support in Linux? How hard is it to rebuild an array? How well does the hot spare work? Will it rebuild using the spare automatically if it detects a drive has failed?"

Comments

Re:Advice: Get lots of RAM

[mortonda]

The idea is that in order to write data to any sector on one of the drives, the sectors from six of the other drives need to be read, all XOR'd together, and then the result written to the remaining drive.

Your logic eludes me. The blocks do not need to be read, as we are in the process of writing. We already have the data, because we are writing, so why would we re-read the data?

Furthermore, block sizes default to 4k, though you could go to 8k or 32k block size. At any rate, you don't need a gig of RAM to handle this.

Finally, XOR is not that expensive of an operation, and a 500Mhz CPU is going to be able to handle that faster that any but the most expensive controller cards.

So unless you are actually a RAID kernel developer, I don't buy your story.

Re:Here is a better question

[GigsVT]

I just posted in another thread about 3ware and mysterious drops of seemingly good drives. Even with the ultra-paranoid drive dropping, we have never lost data on 3ware.

Other than that, 3ware has been decent for us. We are about to put into service a new 9500 series 12 port SATA card.

I wish I could say our ACNC SATA to SCSI RAIDs have been as reliable. We have three ACNC units, two of them went weird after we did a firmware upgrade that tech support told us to do, lost the array.

We call tech support and they say "oh we didn't remember to tell you when you upgrade from the version you are on, you will lose your arrays".

Re:Works great

[k.ellsworth]

Normally a drive crash anonunces itself some time before... use the smartctl tool.
that tool checks the SMART info on the disk about posible failures..

I do a lot of software raids and with smartctl, no drive crash has ever surprised me. i always had the time to get a spare disc and replace it on the array before something unfunny happened.

do a smartctl -t short /dev/hda every week and a -t long every month or so ...

read the online page of it:
http://smartmontools.sourceforge.net/

A example of a failing disc:
http://smartmontools.sourceforge.net/exampl es/MAXT OR-10.txt
a example of the same type of disc but with no errors:
http://smartmontools.sourceforge.net/exam ples/MAXT OR-0.txt

Software raid works perfect on linux... and combined with LVM the things gets even better

Re:Stick with hardware RAID

[pjrc]

Consider--your ATA RAID controller dies three years down the road. What if the manufacturer no longer makes it?

This happened to me. The card was sorta still working... could read, with lots of errors usually recoverable, but writing was flakey.

Luckily, even after about 3 years, 3ware (now AAMC) was willing to send me a free replacement card. They answered the phone quickly (no long wait on hold), they guy I talked with knew the products well, and he had me email some log files. He looked at them for about a minute, asked some questions about the cables I was using, and then gave me an RMA number.

The new card came, and my heart sank when I saw it was a newer model. But I plugged the old drives in, and it automatically recognized their format and everything worked as it should.

This might not work on those cheapo cards like Promise that really are just multiple IDE controllers and a bios that does all the raid in software. Yeah, I know they're cheaper, but the 3ware cards really are very good and worth the money if you can afford them.