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.

You can only use two drives with RAID 1.

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.

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.

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.

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.

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)

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.

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.

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.

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.

I think what your'e talking about is RAID 0+1, which can use odd #s of drives.

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.

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.

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.