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Two HDD set up as Raid 1

Discussion in 'Hardware' started by desertwind, Jan 25, 2007.

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  1. desertwind

    desertwind Thread Starter

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    I've been looking up info on setting up a RAID 1 array. If I understand how this configuration works, if you have two 80gb HDDs in a RAID 1 setup, basically you have a total of 80gb of storage with one of the drives acting as a mirror or backup, correct?

    Assuming this is the case, this would also require that both HDDs be of equal capacity? I used 80gb in the example above. What happens if one drive is 160gb and one 80gb. Does it simply stop mirroring data after the capacity of the lower drive is reached or can it not even be set up with mismatched drive capacities?

    I like the idea of redundancy in case of a crash, but wonder how important this really is? Most of my user data will fit on 3-4 CDs.

    Thanks!

    Tawni
     
  2. Kustom

    Kustom

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    Well RAID 1 is ideal if you are running a server or are running a home business etc.. Personally i never see the use for it I always set up Raid 0. First you will want to figure out what raid controller your motherboard is using, if it is using a silicon valley controller or your chipset. Chipset manufacturers usually have a software program downloadable that will help you understand and configure your raid through your OS but you will need to look for the insturctions on how to set it up in your BIOS utility first.

    More times than not it is really easy usually accessed by pressing ctrl+i to enter the utility. Also make sure you enter your BIOS and check the settings in there, you should be able to find a reference guide online depending on your BIOS manufacturer.

    As for the 160+80gb hd question. You could set up a RAID array on those two HD's up to a maximum of 80gbs.
     
  3. JohnWill

    JohnWill Retired Moderator

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    Don't get thinking that RAID-1 is backup, because it's not! IMO, there is no good reason for a home user to ever consider RAID-1.

    A MUCH better idea would be to use one of the disks for the system, and use the other one for backups. Then you REALLY have two copies of your data, and you REALLY have backup. ;)
     
  4. Kustom

    Kustom

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    Agreed I would not recommend a RAID-1 to anyone, I would recommend setting up a RAID-0 with a ghost partition.
     
  5. crjdriver

    crjdriver Moderator

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    If you are going to setup raid, it is best to use drives of the same size, speed, and cache. While it is possible to use different size drives, it can lead to problems.

    If you are going to use raid, then you should have a third drive that is not a member of the array to backup your data and or an image file of the array.

    Raid1 is helpful in the event of a drive failure; it does nothing for a virus, bad driver install, etc.
     
  6. jmtechserv

    jmtechserv

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    Mirror drives are OK as long as you keep in your head that
    The only thing you are supplying redundancy to is the primary
    Hard drive.

    It is however, as industrial strength BAD idea to mirror two drives
    That are not identical in size, density, and composition.
     
  7. desertwind

    desertwind Thread Starter

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    Thanks for the response, guys. I guess the thing that I don't understand is the difference between "redundancy" and "backup", since both are replications of data. Since drives have gotten to the point where their capacity is large and their cost relatively low, it doesn't really seem practical or efficient to try and back data up on anything but another HDD.

    Actually, though, my data files probably represent a small percentage of actual physical disk usage and could be backed up by transferring to CD or DVD disks. I just thought a passive form of backup sounded very convenient, which is why the RAID 1 configuration appealed to me.

    Tawni
     
  8. Kustom

    Kustom

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    I think the best way to think of it is that RAID 1 will prevent a Hardware failure wheras if you have a seperate partion with a ghost image it will prevent software failures.
     
  9. apr911

    apr911

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    Raid 1 = Redundancy which is basically saying everything you do is immediately done on both drives. This is great if you are concerned about hardware failure and a subsequent loss of data.

    Ghosting = Backup which is comparable but much more advanced to Windows XP System Restore. It allows you to go back to the way your computer was at the time of the backup. This is great if you are concerned about software (windows) failure due to a variety of reasons and its subsequent data loss.

    Of course windows corruption does not always mean you will loose your files, but it does make recovering them more difficult and you will likely lose system configurations and software installs in the process off fixing the corruption.

    Saving the backup images on a partition of the same drive will not help you in the event of hard drive hardware failure as your backup is saved on the same physical drive as the one that just died.

    As already mentioned, it is recommended that when establishing a RAID configuration of any sort that you use Hard drives of the same size, speed, cache, density, composition, etc. Usually this is best achieved by buying 2 of the same hard drive. This is recommended procedure, however it is not required. Good hardware based raid should have no major issues with the different drives however, for most non-server level RAID cards, the raid may run into issues. Raid Configurations setup in mismatched drives will always follow the smallest drive size and the remaining drive space on the larger drive is useless. In the event that you are able to partition the extra space on the larger drive it is recommended that you dont as doing so can cause your performance to decrease due to limitations by the drive in data handling and the necessity for the raid card to inspect files being saved to determine if they need to be duplicated into raid or saved on the one drive.

    Another option is to use your raid controller to do whats called JBOD (Just a bunch of discs) or alternatively called concatenation. This allows you to combine your 160gb hdd and 80gb hdd into 1 logical 240gb hdd. This option has the same limitations and downfalls as raid 0 which are discussed below and none of the performance increase. In your case this is not a recommended option since you are starting with medium/large discs already, this is most advantageous to users with multiple small drives (likely under 40gb) who are looking to create 1 big drive out of them

    Your final option is to go to a higher level raid but these will require more than 2 discs and are likely not supported by whatever RAID controller you already have. Furthermore, they serve no practical purpose to the home user. These configurations are designed for servers where it is important that the hard drives perform faster than RAID 1 but still have the redundancy/reliability of RAID 1.

    RAID 0 is not considered to be true RAID because it is not fault tolerant and falls outside the definition of RAID which is Redundant Array of Independent Discs since there is no redundancy involved in RAID 0. The people stating that they dont know why you would use RAID 1 and recommend RAID 0, consider this Yes Raid 0 is faster than Raid 1 and single disc configurations (this speed improvement is likely not to be noticed by most home users) but since all discs need to be operational in order for your system to work you have just exponentially increased your potential rate of failure.

    To show/prove this, lets assume a hard disk has a 10%* chance of failure in the first year, 25%* in the second and 50%* anything beyond year 2.

    On Raid 0 with 2 discs: (1 - (1-rate of failure)*(1-rate of failure))*100
    Year 1 Chance of system failure = 1-(0.9*0.9) = 19% Failure
    Year 2 Chance of system failure = 1-(0.75*0.75)=44% Failure
    Year 3+ Chance of system failure = 1-(0.5*0.5)=75% Failure

    On Raid 1 with 2 discs: (rate of failure * rate of failure) * 100
    Year 1 Chance of system failure = (0.1*0.1) = 1% Failure
    Year 2 Chance of system failure = (0.25*0.25) = 6.25% Failure
    Year 3+ Chance of system failure = (0.5*0.5) = 25% Failure

    Single Disc non-Raid Configuration: (rate of failure)*100
    Year 1 Chance of system failure = 10% Failure
    Year 2 Chance of system failure = 25% Failure
    Year 3+ Chance of system failure = 50% Failure

    So you can keep your less reliant Raid 0... Ill stick to my Raid 1... Mind you, I still do Backup images to avoid software induced data loss...


    * Failure rates are not exact or represented as fact and are assumed. However, the equations will still show the same Raid 1 to be most reliant, single disc non-raid to be second most reliant and Raid 0 to be the least reliant configuration regardless of actual failure rates. Try it!
     
  10. Kustom

    Kustom

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    I have never seen any factual evidence to support his claims about how reliant the different configurations are.
     
  11. apr911

    apr911

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    The percent rate of failure is not represented as fact as I do not know the actual percent rate of failure for hard drives especially since every hard drive make/model has its own percent rate of failure.

    However, the equations do still hold true regardless of actual percent rate of failure of the hard drive.

    Because all drives must be operational in order to maintain a working system in RAID 0, your failure rate increases in RAID 0 vs Single Disc Non-RAID because you have increased the possible points of failure to the number of drives present in the RAID array.

    Because only 1 drive must be operational in order to maintain a working system in RAID 1, your failure rate decreases in RAID 1 vs Single Disc Non-RAID because the likelihood of loosing all drives at the same time is less that of the likelihood of loosing a single drive at any given time.
     
  12. JohnWill

    JohnWill Retired Moderator

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    Of course RAID-0 has a lower MTBF than RAID-1, but you're considering only the hardware aspects of the equation. Since you're a lot more likely to lose data from software or operator issues, that's not exactly a smart way to view the problem.

    I'll say it a final time: RAID of any level is NOT backup. :rolleyes:
     
  13. Kustom

    Kustom

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    Im with John on this one. Best possible set up in my opinion would be a raid 0 stripe with a seperate ghost partition. If you set that up your perfomance will increase you will be happier and if one of your HD's does by some chance fail you have that ghost partiton to bring it back.
     
  14. desertwind

    desertwind Thread Starter

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    That does seem to strike a balance between performance and redundancy. I believe some refer to this as RAID 0+1? I will consider it. I still feel more comfy having my most critical files saved on removable storage media in multiple copies.

    Tawni
     
  15. apr911

    apr911

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    I am aware that RAID is not a backup and I did not advocate it as such. Read my original post. My post was designed to better define redundancy and backup while at the same time illustrating to desertwind the options he has for setting up any sort of RAID configuration with his current 2 hard drives. My second post was to defend my position against Kustom's statement of having heard no factual evidence to support my claim.

    That being said, I think it is important to maintain a ghost image. Myself personally I run a RAID 1 configuration with Ghosting, I find this to be the best setup because the I maintain a backup for Software related data loss while maintaining redundancy. This way in the event of HDD failure, I have no downtime what so ever. I dont even have to wait till I purchase a new hard drive to repair the array, i can run of the one disc. Also my RAID controller supports NCQ and the ability to read from both discs simultaneously therefore the performance increase seen between RAID 0 and my RAID 1 configuration is minimal and as a non-server home user is not really noticeable.

    My point was that RAID 1 is more reliable and likely preferable to RAID 0 because of the significantly increased reliability vs marginally (for the home user) increased performance of RAID 0.

    Especially since many RAID 1 cards now days support reading from all drives simultaneously, thereby increasing its read performance to be quicker than a Single Disc Non-Raid config.

    Finally, RAID 0+1 requires more drives since it is doing 2 processes... Raid 0+1 is a striped array of mirrors which is to say it performs RAID 1 on a set of RAID 0 discs. Which means you need 4 drives at which point your likely better to go to a higher RAID configuration that contains parity info or a RAID 3 configuration which only requires 3 discs

    Kustom advocates RAID 0 with a seperate ghost partition I am assuming that he means a seperate ghost drive since it is not advisable to create a non-raid partition on a raid drive and if you save the image to a raid 0 drive pratition if any of the drives fail, you loose your ghost image.
     
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