On 07/02/13 17:07, Jean-Francois Simon wrote: > Le 20/05/2013 13:46, Nick Holland a écrit : >> On 05/20/13 00:52, Hugo Osvaldo Barrera wrote: ... >>> 3) The man pages report RAID5 as experimental. I'm curious, why >>> is this so? Is it just not-very-thoroughly tested, or is there >>> some missing feature? I read on a 2010 presentation that rebuild >>> was not implemented yet, is this still so? >> That's really a question you will need to find out though >> experimentation before you implement (i.e., you MUST practice this >> recovery stuff before going into production), but yes, RAID5 >> rebuild is still not there, so I would NOT recommend going this >> route. >> >> However, a nice little RAID1 system to start, hopefully leaving you >> two SATA ports for the next generation/upgrade disks. >> >> Nick. > > "RAID5 rebuild is still not there" Can you please make it more clear > what actual state of soft raid can and what it cannot do under RAID 5 > ... I'm not so sure to get it, thank you. > > J.-F. >
"RAID5 rebuild is still not there" -> there's no RAID5 rebuild. I'm not sure how to make it more clear... Ok, let's try this... Today, you take four 1TB disks, and make a 3TB RAID5 volume. You can do that. Works great. Now, a lot of people might call this "Job Done". Not me. The point of RAID isn't to build complicated systems, but to have the system keep your butt out of the fire when things go wrong. Next month, one of those drive fail. That's ok, RAID5 is designed to keep your data usable with one drive down. THAT is the point of RAID. You pat yourself on the back and say, "I'm glad I am using RAID5". You replace the failed drive and... ... um... now what? You have a three drive degraded RAID5 system with no remaining redundancy...and a new drive that is currently unused. You have no ability to rebuild the function of the failed drive into the new drive...because the RAID5 rebuild is not there. Oh, poo. Your options? Well, * you can build a NEW array on other disks (hope you have enough ports to plug them into), copy the data from the old one to the new one * you can hope your backup system is perfect, and rebuild the entire array and reload from backup * you can hope a second drive doesn't fail in your array... for the life of the system. Not much else I can think of. If you want to play with softraid and raid5, hey, have a blast. You want to put critical data on it? I'd not suggest that. A job ago, I had some relatively large chunks of data to hash through to find some needles of data in and no disks handy that could do it in one chunk...but I had some big disk array boxes, and a lot of smallish SCSI disks I could stick in them (and the office space was really cold, so a bit of heat under my desk was not unappreciated). I think I did them as softraid RAID0, but I could have done it as RAID5 with this system -- the data is there just for analysis, not storage. RAID5 might give me a few minutes to pull data off that I realized was important only after the drive failed, but otherwise the loss of data on this array would not have been catastrophic at all. Now, anyone who drops important data on any kind of RAID system without figuring out how to deal with disk (and controller) failures deserves what they get. So if I was a nice guy, I'd have said "Go try it out on some spare hardware and unimportant data and answer your own question", but being the evil bastard that I am, I'm denying you a very important learning experience. Nick.
