RAID 1: Mirroring and Duplexing. For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair.
RAID Level 1 requires a minimum of 2 drives to implement.
RAID Level 1, also called mirroring, has been used longer than any other form of RAID. It remains popular because of its simplicity and high level of reliability and availability. Mirrored arrays consist of two or more disks. Each disk in a mirrored array holds an identical image of user data. A RAID Level 1 array may use parallel access for high transfer rate when reading. More commonly, RAID Level 1 array members operate independently and improve performance for read-intensive applications, but at relatively high inherent cost. This is a good entry-level redundant system, since only two drives are required.
Advantages: One Write or two Reads possible per mirrored pair. Twice the Read transaction rate of single disks. Same write transaction rate as single disks. 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk. Transfer rate per block is equal to that of a single disk. Under certain circumstances, RAID 1 can sustain multiple simultaneous drive failures. Simplest RAID storage subsystem design.
Disadvantages: Highest disk overhead of all RAID types (100%) - inefficient. Typically the RAID function is done by system software, loading the CPU/Server and possibly degrading throughput at high activity levels. Hardware implementation is strongly recommended. May not support hot swap of failed disk when implemented in "software". Recommended Applications� Accounting � Payroll � Financial � Any application requiring very high availability.
RAID 1 isn't a substitute for backup because there are a lot of risks that it can't protect against.
If you accidentally delete a file, it will instantly be removed from both mirrored copies.
If your disk is corrupted by a software bug or virus, the corruption will be done to both mirrored copies simultaneously.
If you're hit by a bad enough power surge, it'll probably fry both disks at the same time.
If someone breaks into your house, they'll steal the box that holds both disks.
If your house gets flooded or burned, both disks will be ruined.
Don't discount these risks! I don't use RAID, and I've never lost data due to a drive failure. But I have lost irreplaceable home videos when my house was broken into (back when blank VCR tapes were $20 each). I wish I'd backed those up...
Also, i would like to add failures of the RAID-layer itself.
In my estimation, most windows (fake)RAIDs do not fail because of a disk failure, but a failure on the RAID level instead causing a broken array. The user may not be able to recover from that situation, even though the data is still physically on the drives; the user lost access to it. Any hastily action the user takes may destroy the data permanently.
So for all intents and purposes, a RAID-array should be seen as a single disk; it can fail. That's why you need a backup.
I have used Raid 1 mirrors for about 15 years now. The are great when a drive decides to go south. Put in a spare and it rebuilds and you are ready for the next failure.
Or keep an extra drive in the box designated to the RAID controller as a spare, like I do and it does it automagically.
As Sub Mesa said, the RAID layer can cause problems. One of the two catastrophic failures I had was when the RAID controller went flakey. A drive died, it started the rebuild to the spare and when it was done both disks were trashed. Unbootable, random directories gone, totally hosed. Seriously screwed if not for backups.
The other was a recent issue where the rebuild would fail and trash the MBR on the original drive. (This one was just a BIOS RAID with the Intel Manager). It turned out to be an issue with the drive firmware, but again, screwed if not for backups.
So many good, fast easy to use tools out there. No excuse for not having a bootable repair disk and good backups.
I'll second sub mesa and Scarabiii's comments and add that the user is also a weak link in the chain. For example, I wonder how many recoveries from failed drives have been botched by removing the wrong drive for replacement?
From the tone of the posts here I get the impression that people go in and configure RAID arrays without spending any time testing their recovery procedures. That's asking for trouble!
When a drive dies and your data is in the balance the last thing you want to be doing is learning how your RAID controller works by trial and error.
RAID1,5,etc will ONLY protect you against a hardware failure, NOT data corruption,etc. Imo, you should ONLY run RAID0 for OS/scratch disk, and if running RAID 1, you MUST STILL have a good back up plan.
The best back up plan would make 3 copies, for example, I back up all my files on to a USB HDD and NAS. I put important files (ie pics, CAD files, etc) on to BD/DVD (kept in a fire proof safe) and then encrypt/compress and back them up online (I pay $ 10/mo for 50GB).
Note: All important files are encrypted and tested before and after back up.
[quotemsg=5962140,9,403977]To make a backup, remove one of the disks and replace it with a blank disk. The disk that you removed is your back up.[/quotemsg]If you're going to do that you'd better actually test it to ensure it works. I've worked with RAID controllers that didn't allow me to create a RAID-1 set from a single drive with data on it. I could take the single drive that I'd broken from the set earlier and use it to boot from, but there was no way to get it back into a RAID-1 set again without destroying the data on it.
In my experience RAID controllers tend to have unexpected quirks like this, so it's very dangerous to assume they'll be able to do something unless you've actually tried it and confirmed it for yourself.
external enclosure are poor for data backups, they are not reliable and RAID1 rocks FTW!!!!
your arguements that RAID1 is not suitable for backup are a bit weak in my opinion, firstly, using your logic, ANY backup solution is not suitable because you need 3 different copies. The chances of a power surge are quite small especially. NAS/ext boxes heat up which limits the lifespan of the product and reliability. Software corruptions can happen anywhere. And yes, if you delete the stuff on one drive, yes it does delete on the other, but who's the moron playing around in the backup drive deleting ***??
[quotemsg=5962570,12,201253]your arguements that RAID1 is not suitable for backup are a bit weak in my opinion[/quotemsg]All I can tell you is that in my 30 years as a data centre administrator I've never once seen any commercial shop of any size that relies on RAID-1 as it's sole backup strategy. All but the smallest shops back up to external media and store copies offsite.
These guys are spending major bucks on backup, and if they thought they could get away with just RAID-1 then they'd surely do it.
RAID protects against hardware failure only. Does not protect against user error, natural disasters etc. Backups provide you snapshots of your data at any point in time in the past. Obviously, the more copies you keep, the safer your data is.
Do you keep two house keys on your key chain? It will help you if one breaks but that's about it.
[quotemsg=5962766,17,377229]RAID protects against hardware failure only. Does not protect against user error, natural disasters etc. Backups provide you snapshots of your data at any point in time in the past. Obviously, the more copies you keep, the safer your data is.
Do you keep two house keys on your key chain? It will help you if one breaks but that's about it.[/quotemsg]
yes, but unlike a hdd if a key breaks you don't loose your house and contents :/
[quotemsg=5943122,1,359198]why is raid 1 not a substitute for a backup???[/quotemsg]
Raid 1 is not a 'substitute' for a backup BUT a Raid 1 backup is the very best way to safeguard important information.
If you have photos and videos on your machine that you edit and want to back up. Buy a couple of disks and an enclosure, or one of these aftermarket devices, and backup anything important. Now you put that backup in a safe deposit box or in your office desk and, should something terrible happen, you have a backup.
I'd suggest that your data would be safer if you took those two disks you were using for your RAID backup and instead of putting two identical copies of information on them, instead you use each one individually and alternate backups between them.
That way you still have two copies of most of your data, but now if you discover some problem with the most recent backup (like you deleted an important file just before you ran the backup) you can go back to the previous backup.
The more cycles of backup you have, the more scenarios you can recover from.
I had a similar thought, after deciding to add RAID 1 to my machine. I have been through a lot of imaging software (and a few of them actually work), but it suddenly seems like a lot of work....
The plan is to have several e-sata connected external drives, one of which is always connected as the 2nd drive in the raid. Monthly, or so, replace that drive with another. The controller will sense a failure and rebuild.
The volume can be verified easily. the enclosures I bought also have a USB interface. Simply "boot from USB drive" to see if it works.
As a stand-alone method, I have enough bad memories to know better...! In my home I have 7 computers on the network. I have Ubuntu on most of them now, and use Unison to mirror user documents onto other machines. I also do a full user files backup onto another external drive using TrueCrypt, and store it off site.
Does that about cover it? I'd like to hear more about using the RAID 1 as a reliable imaging tool.
Great forum! glad I found ye-