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Old 5th November 2012, 7:53 PM   #1
ex4n Thread Starter
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Default Mirroring 1U twin servers

Hi guys,

Looking at these 1U supermicro twin servers, would it be possible to configure these servers in some kind of mirrored config, similar to raid1 for HDDs.

I assume this is simply not possible at a hardware level, but correct me if I am wrong, and so I am wondering if there is any software that could do something like this?

Effectively both servers would be doing the exact same thing and if one fails, the other takes over until it can be replaced.

Sorry for the fairly vague question, any help or suggestions would be appreciated.

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Old 5th November 2012, 7:56 PM   #2
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You probably want to provide details of intended use, OS etc.
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Old 5th November 2012, 8:01 PM   #3
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thanks gords, that's probably a good idea. it is something a potential client has asked about. He is likely to be running a windows server, not sure what he will be doing with it yet.

I have looked at drdb for nix + heartbeat which could potentially do this well, but I think he's a windows man..

Edit: Will update with more info when I can get it, was just looking for ideas for now.
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Last edited by ex4n; 5th November 2012 at 8:06 PM.
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Old 5th November 2012, 8:20 PM   #4
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Possible simple solution could be:


Install similar apps on each server.

Copy on hobocopy.

Set a scheduled task to hobocopy one server over top of the other.


Suggesting hobocopy as it creates a snapshot before copying, so you can copy appdata etc of running programs. Basically, create a .bat to copy the entire user profile over from one server to another.
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Old 5th November 2012, 8:23 PM   #5
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It's the exact same as setting up redundancy on any two servers, just because they're in the same case doesn't make it any easier. They also share the same (non-redundant) power supply so they will both go down with a power related fault.
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Old 5th November 2012, 8:34 PM   #6
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thanks guys, i've never looked at this kind of redundancy before, usually i would spec a single machine not to fail, such as 2x psu, 2x rc, mirrored disks etc.

I probably could have worded the OP better, I appreciate the options to look into
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Old 6th November 2012, 12:40 AM   #7
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Sounds like Windows Clustering could help, depends on what you're actually trying to do. Little known fact - you can cluster a heck of a lot more stuff than just the normal suspects (Exchange, VMs, SQL). As long as it's Windows Services or you can script it in VBScript/JScript, you can cluster it with health checks etc.

We need to know more about what's what. Data replication or shared storage? Maybe two-phase commit? Intended apps? RPO requirements (how much data on your "mirrored" server can you afford to lose)?

Is it mirroring for availability or performance? If it's availability, what happens when the place catches fire? How do you plan to get a consistent backup if the data is in flux?
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Old 6th November 2012, 2:59 AM   #8
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When the client came to me today, I had assumed he was asking as he had seen/heard/read about this being done somewhere, upon further investigation, it seems he just assumed that it would be simple, like mirroring hdd's. I posed the question to you guys in case there was actually a simple piece of software capable of doing this.

This was for redundancy only, so if one server failed, the other would take over and there would not be a second of interruption. I have explained to him the complexity of achieving something like this, and that there are other, better options for achieving the same thing.. so he is going to get back to me about his intentions.

Sorry I can't be much more specific as he never told me exactly what he was going to be doing with them, will update again if I get more info or any follow up.

It is most likely the idea will be dropped for now.. in favour of a more practical solution

Appreciate the input all.
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Old 6th November 2012, 8:45 AM   #9
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The "simple" option is to use virtual machines on top of your twin servers. There will still be some downtime if one host unexpectedly falls over while you boot up the VM on the other node, but if you know it is coming in advance, you can live migrate your VM to the other piece of hardware.

We have a couple of small clusters set up in this way where 1 node can (just) take all of the load (we suspend any unimportant jobs if we're stuck on 1 node), but usually the two nodes have the load spread evenly across them.

If your application is cluster-aware (or can be made to be cluster-aware), that is usually a better option. For example, if you're doing web-hosting duties, the best option would be to stick two web servers behind a load balancer and synchronise the web files between the two nodes.

Of course, if your budget is unlimited, specialised hardware is available such as the Stratus Continuum products. Even then, that requires some thought at the application level to properly leverage the capabilities of the box.
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Old 6th November 2012, 9:09 AM   #10
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Quote:
Originally Posted by ex4n View Post
When the client came to me today, I had assumed he was asking as he had seen/heard/read about this being done somewhere, upon further investigation, it seems he just assumed that it would be simple, like mirroring hdd's.
Sounds like another case of someone reading some buzzwords in CIO Magazine while waiting in the Qantas lounge, and then telling IT in an offhand comment to "make it happen".

Quote:
Originally Posted by ex4n View Post
Sorry I can't be much more specific as he never told me exactly what he was going to be doing with them
For infinite use cases there are infinite solutions at infinite price points. Specific requirements will give your client a specific answer.
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Old 6th November 2012, 9:10 AM   #11
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Depends on the $$$ we are talking, for simple Mirroring with Auto fail-over you can do this a couple of ways, Active/passive style (one way replication) using something like Double Take Availability as a physical option or using Hyper-V replica to replicate the Virtual Machine to a hot spare (trans log style shipping so potential small data loss as with most/any replica's) but both options have automatic fail-over. Both these options will support Linux OS's (however the Hyper-V option is limited somewhat) (Yes you can use the Free Hyper-V 2012 Core version as its supports Replica too)

As with all replica style services and defiantly with Hyper-V replica there is a I/O overhead that needs to be accounted for but if your replicating a single VM just for the HA factor then you wont need to worry about that.

Next up you have the Clustering for Active/passive or Active/active HA depending on the application type.

Just my 2 cents...
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Old 6th November 2012, 9:36 AM   #12
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If he has some kind of shared storage, you could put in a failover cluster, however a 2 node cluster is troublesome. Something like HyperV or VMware could help if you want to have multiple autonomous systems running on it.

It depends if you want to just replicate the data for redundancy, or actually run applications on the two servers. If you had some kind of plan and costs in mind we might be able to help out more.

Data replication between two servers is certainly possible, with cli things like robocopy, but they are never 100% 'mirrored'. With these though, large file copies are a pain as it needs to replicate the entire file with a minor file change (think email PSTs). Block level replication is better for this kind of thing. Options for this could be DFS-R (Free with Windows 2008) or there is also another product that used to be Veritas but I believe they were bought by another company so the product might be rebranded. It was called VDR or something (been a few years since I used it) It established a relationship between 2 server disks and replicated changes only.
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Old 6th November 2012, 10:45 PM   #13
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Windows Server 2012 should make this type of clustering easier with SMB 3, SAN like replication between servers.

Also server 2012 then can do things like clustering updates, where it copies services over to one server whilst it performs updates on the other.
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Old 6th November 2012, 11:27 PM   #14
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Since we're throwing out crazy ideas I'm surprised nobody mentioned vmware's product called "fault tolerance"..

Rather than try and explain it, I'll just paste the blurb:

vSphere Fault Tolerance (FT) provides continuous availability for applications in the event of server failures by creating a live shadow instance of a virtual machine that is in virtual lockstep with the primary instance. By allowing instantaneous failover between the two instances in the event of hardware failure, FT eliminates even the smallest chance of data loss or disruption

Haven't seen anyone try and gold plate their solution to this degree .. yet.
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Old 7th November 2012, 11:21 AM   #15
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I would suggest something like ESX and Veeam backup and replication.

it would just snapshot and replicate the data between the servers every say 15min-30min so you would only loose that much per sync...
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