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Raid 5


JayF
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I am hoping someone can help I have a pc which I have made into a Media Server Running Freenas and it works really well anyway I have just added 2 more HDD's to the system, so I now have 2 500GB drives and 2 1.5TB Drive, I have configured it into a raid 5. my problem is that the total space is 4TB but it is only giving me 1.3TB of usable space, Am I wrong in thinking that it should not need to use 2.7TB for Rundentcy, I thought I would of had at least 2TB or usable space. Or will I need 4 identical drives, hope someone can help

James

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Traditional RAID 5 requires all drives be of the same size, if the drives are not of the same size the size of the smallest drive is assumed for all drives. Your current drives in RAID 5 will be the equivalent of having 4 500GB drives in RAID 5. One drive of space is lost of redundancy, so (1.5TB - ~20%) 1.3TB is the amount of space that would be expected.

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Raid 5 requires a minimum of 3 hard drives and they all must be equal in size. With Raid 5 you also loose some capacity, due to the parity checks it performs.

So if you have x4 1TB hard drive, you only get 3.3TB of storage just like Sparda pointed out.

In other words, its just how Raid 5 works, you have done nothing wrong.

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As others have said you will need to have disks in the RAID with the same size to avoid problems like those you have encountered. So I would suggest looking at getting another one or two 1.5TB disks to go with the two you have.

You only need 3 for RAID5 but if you do have 4 disks and you RAID supports it I would recommend using RAID6 as it gives you better redundancy than RAID5. You would be surprised at how many times you have more than one disk fail in a RAID setup. Of course like most things in computing it is trade off between redundancy and usable spaced, just remember that you will most likely have to live with what you choose for quite a while.

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As others have said you will need to have disks in the RAID with the same size to avoid problems like those you have encountered. So I would suggest looking at getting another one or two 1.5TB disks to go with the two you have.

You only need 3 for RAID5 but if you do have 4 disks and you RAID supports it I would recommend using RAID6 as it gives you better redundancy than RAID5. You would be surprised at how many times you have more than one disk fail in a RAID setup. Of course like most things in computing it is trade off between redundancy and usable spaced, just remember that you will most likely have to live with what you choose for quite a while.

Raid 6 may give you better redundancy but offers minimal performance, in terms of reading/writing. I would only recommend it, in situations where you would require more redundancy than performance.

However you can still get good performance out of your raid 6 configuration, if you invest into a decent raid controller.

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The read performance of RAID6 is the same as RAID5. It is only the writes that can have a lower performance It needs to do a few more calculations than RAID5 does as it keeps multiple parity blocks. If you have good RAID controller then the extra calculations will be done in custom hardware and RAID6 will be just as quick as RAID5 (the disk writes take up the time not the calculations).

If you are using software RAID though it would be worth taking sometime to think about performance issues and if you want more redundancy or write performance as Infiltrator's post.

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The read performance of RAID6 is the same as RAID5. It is only the writes that can have a lower performance It needs to do a few more calculations than RAID5 does as it keeps multiple parity blocks. If you have good RAID controller then the extra calculations will be done in custom hardware and RAID6 will be just as quick as RAID5 (the disk writes take up the time not the calculations).

If you are using software RAID though it would be worth taking sometime to think about performance issues and if you want more redundancy or write performance as Infiltrator's post.

Unfortunatly Freenas doesnt support Raid 6 at this time not sure it ever will, it is the only os I have found which is free and has UPNP support, I was going to use Ubuntu and use Mediatomb but I found configuring the disks to be a pain in the ass maybe it might have something to do with the fact that I dont know linux a great deal! lol also the drives are in a software raid I dont have a raid controller, thanks for your help :-)

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