Raid question

By DanLM ·
Ok, I am going to admit my ignorance on this subject by asking the following questions.

Let's say I want to build a Raid system for a small server. I have the choice of using software raid or an actual raid card. At the time this raid system is built, I decide that I am only after mirroriing because of the small volume of data which can reside on one physical drive. I decide to go with software raid, i/o access is not a real issue at this point.

So, here lies the question. If I start out using one physical form of raid, say software raid one. And due to the volume of data increase's to the point that this type of raid is no longer a valid option. Can you change over into another form of raid if you purchase the required number of drives, and also possibly a raid card?

As I said, I am truely showing my ignorance here on this subject. I do know there are different types of raid, and from what I have read I beleive that Raid level 5 would be a good solution at a latter time.

But, I am looking to start with a raid level of 1 and then to move at a latter date to a raid level of 5.

Possible? Must be, but how difficult would it be to change?


p.s. I don't have my portable FireFox with me today, so you can all make fun of my spelling(no spell check).

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Although I would recommend

by w2ktechman In reply to Raid question

a RAID card (raid 1 cards are cheap), it is not a requirement. Do not forget that all disc access will be slower using a SW solution.

But saying that, I am not a RAID expert, but I believe that you can break the mirror and setup a L5 RAID later. Although it may depend on the card manufacturer, and what SW they have, I dont think that you can just switch it. I think that it needs to be broken, and then the new RAID setup.

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I rethought my software raid after I walked away

by DanLM In reply to Although I would recommen ...

And felt that a card would be much better, and actualy that is because I want to set this uup as a VM server running linux as the base core os with 2003 microsoft as the layered os.

But, thank you. That's kind of what I thought, that I would have to break the raid to move it over. I was doing some number crunching, and a single drive would meet the requirements of storage. Because the information is important to the company, the raid 1 solution for mirroring onto another drive. I just can't justify going to a raid 5 at this time, but I need to allow for this in the future for when expansion is required.

Again, thank you.


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by cmiller5400 In reply to I rethought my software r ...

If you use RAID1, you can usually break the mirror. RAID5 you usually can not reverse. It is usually a copy/backup data and then rebuild the array. If you have the $$, all you need is 3 drives to get RAID5 going. Some of the newer cards let you expand the array with new drives as you need them.

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by mhobeych In reply to Depends

In most cases S/W RAID gives better performance than H/W RAID, since most servers / PCs runs close to 3GHZ and RAID controllers runs at 300MHZ.

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Since the Data is Important you still need to have a DR Plan in place

by HAL 9000 Moderator In reply to Raid question

A RAID Array is no replacement for any Backup Plan as it's still vulnerable to damage or failure and the potential loss of data.

As far as moving from RAID 1 to 5 you can but you need to break the Array and reestablish it once the new RAID Array is installed or setup and then you restore your original data from the Backup. While In Theory it should be possible to move a RAID 1 to 5 as you have said it's the data that is important not the actual hardware so honestly it's not worth the risks involved.

There is another thing to consider with RAID Cards the Promise Tech ones will allow you to rebuild the Array after a HDD Failure where as others do not offer this option so you have to replace the failed HDD and then reestablish the Array then restore the Data.

I hope that is of some use to you.


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It was very helpful Col, and I appreciated the input

by DanLM In reply to Since the Data is Importa ...

I was not counting on the Raid to be the complete DR plan, this would only be a layer 2 I guess of it.

Backups would be occurring with tapes being moved off site. The fact that if a machine goes down or god forbid, the building is destroyed with everything in it requires much more then just a raid system.

But, from everything that I have read. And please excuse me if I am incorrect, a Raid system will provide me/us with two things. Up time even with a harddrive failure, even if access is slower. And higher access speeds when everything is working as planed.

I was looking at Raid though for the continual functionality if a harddrive failure occurred. With the down time occurring after hours when repairs were/are made.

The next thing I need to do is find articles on using raid under VM. That is the second part of what I have to work out.


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Dan that will work fine

by HAL 9000 Moderator In reply to It was very helpful Col, ...

Though I'm lazy and prefer to have a backup Boot drive available and a Hot Swap SCSI at that as it's an easy and fast fix in an emergency besides I'm lazy.

Any Mirrored Array will be beneficial but unfortunately I have no experience with using one under any form of VM I would however suppose that a Software Array would be the better option there as a Hardware RAID would have to be integrated into the Base OS, but I'll have a think on it.

Generally speaking you get faster access with striped arrays and greater reliability with mirrored arrays on the few times that I actually use stripped arrays I always have a mirrored server to take over the job of the main one if it goes down and have them on different power lines and UPS's just in case something nasty happens.

I'll have a think about running a RAID Array on VM and see what I can think of. Get back to you on that one.


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Sorry but I can't find anything on running a RAID in a VM situation

by HAL 9000 Moderator In reply to It was very helpful Col, ...

I can only think of my original thoughts of using a Software RAID Array in a VM situation as any Hardware RAID would have to be integrated into the BASE OS and you couldn't incorporate it into the VM as well.

But you could incorporate a Software RAID into the VM Subsystem and have it work for that OS.


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hmmmmm, i have to think on this.

by DanLM In reply to Sorry but I can't find an ...

Thank you Hal, very much appropriated. Sorry it took me so long to get back. I've been side tracked on other things.


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Just found this reference to Raid on VMWare

by DanLM In reply to Raid question

And I was thinking of using VMware actually for the our virtual machine software. But, in the setup notes they have posted is the following comment:

Allocate storage and network adapters to be used by the console operating system and virtual machines on the server. Be sure that both the console operating system and the virtual machines have access to at least one device in each category.

Storage: A SCSI or RAID adapter should be shared if you want to use that adapter or array for both the console operating system and virtual machines.

When you are allocating SCSI or RAID devices, the unit of device allocation is a PCI card device. You may connect multiple SCSI or RAID disks, CD-ROM drives, tape drives and other devices to the SCSI or RAID adapter.

You should give as many SCSI or RAID devices to the virtual machines as possible to ensure that the majority of your mass storage resources are used by your virtual machines. If you do not have any IDE disks, you may have to allocate at least one SCSI or RAID device to the console operating system, since the console operating system needs to have a disk from which it and the VMkernel can boot.

Note: If you are installing ESX Server on a Dell PowerEdge 8450, you must assign the on-board Symbios controller for use exclusively by the virtual machines.

Some adapter cards have multiple functions, which means there are multiple adapters on each card. When you allocate a SCSI or RAID device to the console operating system or to the VMkernel, you are effectively allocating all the SCSI or RAID disks, CD-ROM drives and other attached devices along with the adapter. As a result, you have only coarse-grained control over how you allocate SCSI and RAID devices.

Consider this example: Suppose your machine has SCSI adapters vmhba0 and vmhba1 that are on the same SCSI adapter card. If you choose to share one of the adapters, you must share both. Similarly, if you choose to allocate one of the adapters for use by virtual machines, you must allocate both for use by virtual machines.

I really didn't read father then that, but it looks like both vmware and all operating systems can access a raid unit. this looked to be a card. Guess that answers my question then, yes Raid will work with virtual machines and the different operating systems. If the drivers are supported I would imagine.


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