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Basics of RAID
Doggie Fizzle writes "RAID has been common in business environments for ages, and is now becoming more viable and popular for personal computers. This article focuses on the the basics of RAID, and spells things out for beginners or tech veterans. From the article: 'The benefits of RAID over a single drive system far outweigh the extra consideration required during installation. Losing data once due to hard drive failure may be all that is required to convince anyone that RAID is right for them, but why wait until that happens.'"
That's an awful lot of ads for a re-hash of well-known info. Are the editors sure this is frontpage worthy? It looks like a blatant attempt to get page views to me.
http://raid.com/
http://www.killsbugsdead.com/
There's an excellent guide to RAID levels (with pretty diagrams and such) at http://www.acnc.com/raid.html
A source of information with far better content, that isn't simply an excuse to sell ads.
Wikipedia
Depending on the your budget here in the UK you can get an 80Gb HDD for around £35, so split over some time you should be able to afford two (or an extra one if you already have one). This is a good enough reason for anyone to try RAID.
I myself currently have it setup to mirror my data across two 80Gb drives... Four months ago one of the hard disks died (funny buzzing sound, no access) but the manufacturers three year warranty was still valid, so I returned the drive to them for a free replacement. I received the replacement drive and shoved it in, mirrored the data back onto this new second drive and continued as before. If I hadn't have had this setup that data could have been permanently list. It also saves me from writing ten DVDs to store that much.
http://en.wikipedia.org/wiki/Redundant_array_of_in dependent_disks
Seriously, SATA hotswappable RAID 5, put an onboard controller on next gen motherboards, I dont care if its crappy compared to an expansion card, and you will have my money. Yeah we have RAID 0, 1 , 0+1, but no onboard commercial RAID 5 solution in mainstream motherboards. I know its more expenisve, but its also more efficient, and with every failed HD common users encounter the market gets bigger.
There is truth in humor.
Here is a link that explains the basics of computer hardware; I think that it's a good companion piece to the RAID article: http://www.angelfire.com/rings/judy_patch/
Okay I guess it appeals to geeks and fancy computer modders and all. But really, when it comes down to it, a decent main hard-disk, a tray in the second bay for backup hard-disks, and a reasonable backup regimen that people keep up is all a "personal" computer user needs.
Personally, I have 3 backup hard-disks, one that keeps a "clean" base system that I update every 6 months or so, and 2 that I do full differential backups on every 3 days. The "clean" hard-disk is kept off-site, and a script tells me when to do the backups on the other 2. And for very very important files, I just write them on a CD on the spot.
With that, I've yet to lose a single file since I started using Linux in 93 or 94. My solution is cheap and doesn't involve fancy raiding. And I'm quite sure I overdo it, most people could do just fine with one main hard-disk, one backup hard-disk and a little discipline.
"A door is what a dog is perpetually on the wrong side of" - Ogden Nash
With RAID, you still have a single point of failure. Instead of it being your hard drive, it is now your RAID controller. So what is the advantage?
Since a RAID controller doesn't have moving parts, is it less likely than a hard drive to fail?
Don't forget that Friday is Hawaiian shirt day.
IDE HDD Talking to IDE Controller:
HDD: I'm gonna need more time for that write
Contr: Yeah OK, go ahead good buddy
Contr: What's up?
Contr: What's up?
Contr: Error: Drive controller timeout error
SCSI HDD Talking to SCSI Controller:
HDD: I'm gonna need more time for that write because I found a bad block
Contr: Yeah OK, go ahead and remap that bad boy
Contr: What's Up?
HDD: Need more time to map that bad block
Contr: Yeah OK, go ahead
HDD: All done, grabbing the next command in the queue
... but how often do personal backups actually happen? I'm one of those guys that has been taking home backups seriously for a long time, and has a collection of obsolete tape units to prove it. And backups still do not happen often enough if it requires me handling tape.
Let's face it, discipline is a drag, that is why at work IT people are paid to schlepp around stacks of locked cases full of back up tapes to be shipped off site.
So... for my home file server, I went to RAID mirroring, with a 3rd drive in a drawer. A mount-copy-umount chron job copies to the drawer-drive. Drawer-drive gets swapped and taken off site "when I think of it". Because... RAID only protects you from falling over hard drives. It does not proctect you from:
1) Ooops, I wish I hadn't deleted that.
2) Gack! My house just burned down! And took 10 years of tax data with it!
3) Power supply goes wonky, causing both drives to scribble random scorfulentness everywhere.
A home RAID system does not need to be expensive. Who needs hot swap? Use cheapo PATA drives. A few hours of down time for the wife and kids is OK. It doesn't take a big, bad CPU, and software RAID works great.
For those who have run out of internal space in their boxes, and who don't have external SATA or expensive hardware boxes, you can run RAID over Firewire.
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The problem, however, is that out of the box Windows refuses to "promote" an external disk to dynamic, which is required on all post-NT4 rigs for RAID.
The solution is to add a semi-documented Registry flag, EnableDynamicConversionFor1394
HOW TO: Convert an IEEE 1394 Disk Drive to a Dynamic Disk Drive in Windows XP
Couple that with a cheap 4-bay firewire JBOD box and any spare old enclosures and you are set!
I run 2TB in various RAID configs on my Windows server (main and near-line storage). Have done so since 2002. No problems with the external boxes. The support for external firewire RAID is a little gnarly in Windows 2000 - volume must be mounted as a named virtual directory and cannot be mounted as a letter drive. Later Windows give you both options.
Da Blog
I worked for years in development of RAID solutions for a major manufacturer. One of the problems with selling RAID solutions is the lack of understanding, or the prejudice and bias of the people who were supposed to be specifying and buying the hardware.
The 'tutorial' of the parent article is talking in kindergarden terms, oversimplifications and obsolete term, and overlooking some of the issues with using RAID. It's a good example of the true lack of understanding about the subject. By now, there are so many types of solutions that the term RAID hardly applies. But, even 10 years ago companies like Compaq had innovative rudundant storage solutions that were enterprise ready.
Best regards.
No, it's not "Real-Time" but it suits our needs in our home office situation.
I use "Smart Synch" software to incrementally copy the desired directories from the working computers to a "Backup server", an older Celeron machine on the network. Separate partitions are set up for each computer that is being backed up. At Midnight the incremental backups are made.
Then at 2:00 a.m., Smart Synch running on the backup server makes another backup to a USB hard drive plugged into it. That USB HD is on a regular plug-in timer so that it only runs during the time of night when a backup to it is being done. The idea there is that the running time is limited and drive life is extended. Weekly, a backup DVD is burned and stored off site. Am I being anal? Maybe.
"Do the Right Thing. It will gratify some people and astound the rest." - Mark Twain
Ultimately, what it comes down to is that mirroring merely makes the hardware more reliable, it is not a backup technique.
It can be part of this nutritious breakfast^W^W backup technique:
0) shut down the box
1) swap a fresh/new/wiped drive for one of the mirrored drives
2) rebuild the RAID
3) store the just-pulled drive appropriately (e.g. off-site) along with a second identical RAID controller
Now if the machine goes completely belly-up (as in a fire) the user can install the secondary RAID controller and the data-laden drive in a fresh machine, add another fresh/new/wiped drive, and rebuild the RAID in the new machine. This may not be terribly convenient nor perfect for everyone but it will be effective.
Remember, kids: just because a particular technique doesn't perform a task all by itself (in this case RAID 1 != backup) that doesn't mean it can't be part of a larger picture.
I want to drag this out as long as possible. Bring me my protractor.
If that RAID array is the only place you are storing all those home movies, I highly recommend you take a backup to some other kind of media. As other people have said, the RAID controller is a single point of failure. If you lose that, you lose the lot. And there's no guarantee that another controller will be able to rebuild it. Sad but true.
Serving Suggestion: Defrost
I've been using software-RAID with ATA drives on Linux for quite some time, so I can comment on the behaviour of an array containing a faulty drive.
/dev/hdc, ATA error count increased from 0 to 1" and that it would be a good idea to check up on the hosts syslog.
/dev/md0 -a /dev/hdc1" the new one into the array, starting to resync it right away.
First off, let me emphasize how important it is to set up proper email notification (or pager etc.) for such cases! If you don't know about the failure, you're certain to get nice phonecalls from affected users.
If you've set up the notification system (smartd and mdadm come to mind), you'll eventually get an email saying something like "Device:
Checking up on the system, you'll find that the average system load has increased substantially, which is due to the system trying to persuade the disk to write to a faulty sector and the software RAID having to compensate, queuing the errors.
Depending on how often the defective sector is tried to be written to, the load can increase to values of 10 and above, rendering the system unusable. This is a good time to halt it and replace the defective drive, partition it and "mdadm
This may sound really horrible, but in practise it's usually less 60 minutes (counting from receipt of the first notification email) until normal operation can resume with such a system. This is assuming you have all spare parts stored somewhere on site.
In genereal, I've found software RAID1 and software RAID5 on Linux to be exceptionally stable. I'm also very happy about the performance, given that all I'm using is a bunch of el-cheapo ATA disks. As for reliability, I'm convinced it can't be beat in the consumers' price range, since I've seen too many consumer grade hardware RAID controllers go down in a swirl when putting more than a light load on them.
In the enterprise, I've seen companies move to software RAID on their Linux systems, because they found out that their only 5 years old enterprise hardware won't be getting any new spare parts anymore, which includes motherboards, CPU and IO controllers. Moving to software RAID on enterprise grade SCSI stuff allows them to move the entire system to another piece of hardware simply by moving the harddisk to it.