Hi,

 

I read in the technology overview (http://www.lime-technology.com/wordpress/?page_id=13) that unRAID does not stripe data across the drives. If there isn't striping going on, how does the parity drive work?

 

For example, lets say I create a file and all of the data gets stored on one disk. Assuming the parity disk isn't keeping a mirror of those files (since one parity disk can't be big enough to mirror the entire set), how is an entire drive recovered?

 

Also, on a side question; can firewire/USB storage be used with unRAID or is it restricted to PATA/SATA?

 

Thanks!

I kind of figured that the parity drive would store bits needed to keep the stripe of bits even (or odd--it doesn't matter). i.e. if you wanted to do parity on all the bits so they are even:

 

drive 1: 0

drive 2: 1

drive 3: 1

drive 4: 1

parity: 1

 

So if you lose any one drive, you just compute how many bits are on the other drives. If there are an odd number, then the missing bit is a 1 (0 otherwise). If the drive doesn't have a corresponding bit because it's too small a drive, then you assume it's 0. If you replace the bit in drive 3 above with a 0, then you need to change the parity drive to 0 as well to keep the number of bits even. At least that's what I assumed the scheme was based on the descriptions--Tom and correct me if I'm wrong.

I'll try and answer this for you.

 

You are correct that there is no striping or mirroring going on.  What happens is you have a parity drive that is first cleared...I believe to all zero's.  Once this is done parity is calculated and written to the drive.  The server essentially reads all the disks and assigns a value to the parity drive of 1 or 0 depending on the sum of the rest of the drives.  Once completed the array is now protected and if you loose one drive it can be rebuilt based of the parity drive. 

 

As of right now you can only use PATA/SATA drives.  I believe in a future release we might see support for external drives.