[EMAIL PROTECTED] wrote: > In continuation of "raidctl -P all in /etc/rc" theme. > > Please, can you explain what difference between RAID mirroring and > CCD mirroring? > > I see that RAID have reconstruction option, and trace parity > condition. In the other side, it need to "raidctl -P all" and wait for > it before start system. I read ccd(4), ccdconfig(4), but they are not > describe what I want to know. > > And, briefly, what difference between RAID and CCD in such fields > like "performance", "what to do after one disk fail", and "what will > be when one disk fail while system running". >
I think you are refering to raid(4) (RAIDframe) vs. ccd(4) mirroring. The way you wrote it, it sounds like you are refering to the generic concept of RAID. ccd(4) mirroring is a very simplistic system. VERY simplistic. If you need to remirror, you do it manually, using dd(1). You have to decide if that is TOO simplistic for your application or if it is Just Right. You will find that ccd(4) will only mount a mirrored set if all the elements exist, so if you are missing a drive, it just won't mount. There's an easy solution to that: stick a second line in your ccd.conf file which just lists a single drive (wd0/sd0), rather than both. If both drives are there, your first line is used, and the second line is ignored, because device ccd0 already exists, and the wd0/sd0 element is already used. Simple is often a good thing. ccd(4) is built into the GENERIC kernel, and it is on the CD-ROM kernel (I think. don't quote me on that without verification), so "life cycle" maintenance is much easier. RAIDframe is a custom-kernel thing, and this makes life much more difficult for later upgrades and reloads. If you are running the system and one disk fails, I wouldn't bet on either RAIDframe nor CCD keeping your system running, or any other software mirroring solution on any OS, especially with IDE disks. Yes, sometimes it works. Maybe you saw it work. I've seen it not work. Be ready for it. I prefer to think of RAID (in general) as a "get back up faster" system, 'specially until you have had a few failures. I've seen lots of SCSI systems which blow a drive and keep running, but I've also seen them blow a drive and come crashing down. Performance? never looked at that. I've used ccd(4) to do RAID0 for performance, but usually, if I'm doing software mirroring, I'm not worried about performance. Nick.
