RAID 1 - Mirroring three drives?

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Bigft64

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Is it possible to set up a RAID 1 system so that it mirrors onto three drives? I've never run or setup a RAID 1 system before so I'm hoping someone knows from experience.

The idea behind this is I'm part of a small company that wants to upgrade to a RAID server/backup system. Ideally, we would have a server with three identical drives all mirrored. One would be taken offsite for disaster prevention purposes and brought into the office periodically to be updated. The other two would sit in the server at all times so that they would backup each other incase one goes down and we don't lose a week's worth or work for example. With this system, we wouldn't be at risk of losing data incase a HD fails and we would also have a relatively updated offsite backup as well. Can this be done? From what I know about RAID, this would be the only system that would allow offsite backup as well as mirrored in-office HDs.
 
As belvdr says, RAID-1 is for a pair of drives. You could set up a RAID-1 with two drives, and back them up to a third drive, which could be an external drive. If you get two external drives, you can do a full backup weekly to one of them (which is then kept off-site), and a "differential" backup to the other one every day. A differential backup copies anything new or changed since the last full backup, but doesn't reset the Archive attribute, so each day you'll get all files that were added or changed since the weekly backup was done. This reduces the number of backups that must be searched to two, the last full and the last differential; there's no need to wonder when a file was created or changed. The differential backup drive can also be a lot smaller, since it will not include files that never change, like application executables and DLLs.
 

darkangelism

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you cant mirror 3 drives, however to do what you want do a 2 drive raid 1 then get a 3rd drive as an external usb drive and use a backup program to backup data to that drive.
 

nobly

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Agree, I'd RAID 1 two of the drives and then have the 3rd one be an image of the RAID, which would be imaged every X days.
 

Techfan

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RAID 1 configuration can be done with 2 HDDs.
If your server hard disks are hot swap what you can do just have a 3rd hard drive the same capacity as the RAID1 and while is working just swap one of the hard drives with the 3rd one. It will copy everything while you are working and in case and when is finished you will have your OS and data on your spare hard drive. In case something happens to both drives you can still use the 3rd one to start your server.
 

leexgx

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depending on how smart or support the RAID HW is you can some times add Spare disks (RAID 1 + an spare hdd) so if an disk fails it auto start to rebuild with the spare disk (norm its used in RAID 5 the spare disk but i think it work on RAID one as well just if the software/hardware supports it)
 

blue68f100

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You can mirror as many as you like, if your controller supports it. I have 4 in raid1 on my NAS's. These are the boot partitions for the OS. So the machine will boot even if 3 drives fail.
 

darkangelism

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RAID 1 configuration can be done with 2 HDDs.
If your server hard disks are hot swap what you can do just have a 3rd hard drive the same capacity as the RAID1 and while is working just swap one of the hard drives with the 3rd one. It will copy everything while you are working and in case and when is finished you will have your OS and data on your spare hard drive. In case something happens to both drives you can still use the 3rd one to start your server.


Thats not a good way to do it, it will work, but its stressful on the drives to be rebuilt like that and a good way to corrupt data.


@leegx he wanted to be able to take a drive offsite for backup, so a hotspare wont help with that.


@blue a 4 drive mirror isnt a raid 1, its a raid 10 or a raid )+1 depending on whether its stripped then mirrored or mirrored then stripped.
 

blue68f100

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a 4 drive mirror isn't a raid 1, its a raid 10 or a raid )+1 depending on whether its stripped then mirrored or mirrored then stripped.

My drives are broken up in to 2 partitions. The first partition on each drive is in RAID1 (mirror, boot section). the second part is in RAID 5 (data redudency) so I do not have a 10. This configuration is widely used in NAS's. They use a watchdog bios. If one drives fail to boot with in 5 min, it moves to the next.

The point is depending on the controller and/or software you can have a mix.

The question was could he have 3 raid 1. The answer is yes.
 

darkangelism

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thats still not a 3 drive raid 1, thats a raid 1 and a raid 5, unless your saying you have multiple drives mirroring the first, but really you still shouldnt be putting more then one array on a drive, cause then you degrade multiple arrays with a single failed drive. Most controllers do not support more then 2 drives in a mirror set.
 

blue68f100

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Its possiable to make a 8 drive RAID 1 if you want, provided your controller supports it. But what a waste of space. It is not recommended to be swapping drives out of a raid 1 for backup. Thats adds undue stress on the whole system.
 

jstall

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There is no limit to the number of drives you can mirror, we commonly run with 3 drive mirrors, the question is do the costs involved justify the expense of doing it.

If your looking for high performance the controller you use needs to be able to make concurrent writes to every disc in the set, so for three disccs the controller needs to be able to make three concurrent writes, same for reading, the controller would need to be able to make concurrent reads for every disc in the set. If the controller you use cannot do this then you will get no peformance increase over smaller mirror set.

It can provide a very high level of redundancy, in that if one disc in the mirror set fails it allows you to maintain a mirror while you swap out the failed disc, although if redundancy is what you want you may be better of looking at a RAID 5 array for a solution. You would not be losing as much disc space as with a three disc mirror set.

Maybe you should look at a NAS/SAN system that can replicate to a remote NAS, most vendors provide some replication capability. All your servers could access the drives (which could be in an array) and the data could be automatically mirrored to the remote array.
 

Bigft64

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Thanks for all the good replies. Good to know that it's at least possible. In response to jstall, yeah, we're looking at getting a NAS. Performance is not an issue with us and the costs are trivial considering you can find 500 GB SATA-II drives for around $100 these days. As mentioned in my first post, our ultimate goal is to marry the redundancy of a RAID-1 system with the ability to keep a weekly or bi-weekly updated mirror offsite. I know with many NAS' that you can attach a USB drive to it and back up to that but the scenario I was envisioning was being able to bring the "off-site" drive into the office, pop it into the NAS which already has two drives set up for RAID-1 in it, and having the off-site drive be updated automatically to then be taken off-site again. So for that short time, there would have to be a RAID-1 array with three drives in it although most of the time it would only have to support 2 drives. So like I said...I know there's the external USB option but to me it would seem that the sheer simplicity of popping in a drive, waiting a couple hours, and then pulling it out is unmatched. Some have mentioned that the stress of doing something like that isn't a good idea but what good is a RAID if simply cloning/updating a drive compromises the whole system??

BTW, the NAS we're looking to get is the Infrant ReadyNAS NV+. From what I can tell it's a feature-rich NAS and supports everything we need. Feel free to comment on it.
 

croc

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According to my manual, (readynas600) you'd have two ways to do this... 1 way would be to set up a two drive mirror with a hot spare... In your case it would be a cold spare most of the time, but once configured the readynas firmware is pretty tolerant... Bring your off-site drive in, plug it into the hot spare slot, wait for it to initialize, and pull one of your active drives... Patiently wait for the mirror to rebuild, and put your off-line active drive back, wait for the mirror to rebuild again (should be real quick...) and your mirror should now be slots 1 & 2 active, slot 3 hot standby... Pull your standby and take it back offsite. A bit tedious, but...

Another method would be to have two drives as a mirror, and a third drive just sitting there, so to speak. When installed and recognized, do a manual back-up (over write) from your mirror to your third drive.

Go to the infrant website and download a soft copy of your manual, while there ask this as a support question. Their support is excellent, registered user or not. You'll probably have a suggestion from them in your email that day. (There is a phone option I believe, but I'm in Aus....)

As to the product itself, I'd love to be able to justify getting the nv+ myself... If I could find a buyer for my chassis, I would. You'll not be disappointed
 

darkangelism

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NAS is not connected via USB its connected over the network. What you could do though is have a system with a mirror and then use norton ghost or acronis true image to image the drive and save it to a NAS unit that you can take offsite.


@timck, did you read that review, it said that drobo wasnt suited for business use, drobo is $500 with no drives which is expensive.

@croc, pulling a drive and rebuilding to the other spare, its really stressful on the drive because it rebuilds everything, the chance of this drive failing is a lot higher, which isnt good for a backup drive.
 

belvdr

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Its possiable to make a 8 drive RAID 1 if you want, provided your controller supports it. But what a waste of space. It is not recommended to be swapping drives out of a raid 1 for backup. Thats adds undue stress on the whole system.

Is that a specialized chip similar to the Intel Matrix or something? Not even my HP EVA's or the EMC's have the ability to do that (or at least not that I have seen).
 

darkangelism

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Its possiable to make a 8 drive RAID 1 if you want, provided your controller supports it. But what a waste of space. It is not recommended to be swapping drives out of a raid 1 for backup. Thats adds undue stress on the whole system.

Is that a specialized chip similar to the Intel Matrix or something? Not even my HP EVA's or the EMC's have the ability to do that (or at least not that I have seen).


I thought the same thing until i did research as no Dell controller, or lsi logic or adaptec controller supports that.
 

belvdr

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The HP MSA units can, but as previously stated why would you?

Well, I guess you could say the EVA's could do that too, now that I think about it. The EVA's have virtual arrays, not really an array across particular physical disks, and so your RAID 1 would span multiple drives.

I agree, it seems a waste to me.
 

croc

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I'm not recommending either method, and personally I'd have to think that transporting a drive back and forth itself would increase the risk of failure. I run my NAS in raid 5, and if the house burns down I've lost the lot.

I have had several USB external drives, their lifetimes aren't usually very long either...

But the OP seems to think that an off-site drive is a good idea, and at $100 for a .5TB drive, well its affordable.

Were I to attempt this I'd use the 2nd method that I mentioned. The infrant platforms are really flexible, in that until you assign a drive to be something else, (ie, part of an array) then its in JBOD mode.

I also personally wouldn't try to make a NAS a boot device.
 

darkangelism

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Well using NAS for offsite is probably the best way because you can plug that into any network and be up, with a hard drive alone, you need a computer to plug it into. But i dont trust hard drives as backup for critical stuff anyway...I will be using on my new server raid 5, then imaged to NAS then use ntbackup system state to tape.
 
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