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Questions about Parity

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Hey Guys,


I am currently trying to upgrade the storage of my unRAID server but i have a few Questions concerning Parity that i can`t find answers to on the Internet.


First off I currently have 2 x 500GB plus 1 x 2TB Data Drives and One 3TB backup Drive. Now i bought two 4 TB drives and now i would like to replace the two 500GB drives with one 4TB and expand my Parity with 4TB so that i have enough storage for backup.

Currently I am copying all the data from the two 500GB Drives onto one 4TB drive with rsync.


So my first question is: Is it possible to run these Backup drives together without using a RAID 0 ? as i understand if i add a second Parity drive this drive will Backup the first Parity drive but i do not want that. 
Second Question is based of another Forum post here and this Person had 4TB of Data and 2TB of Backup. Is unRaid compressing so that is fits or will he stop to backup after 2 TB of Data? and if he compresses how much Backup storage do i really need?

Thank you Guys in advance.

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I think you misunderstand how parity works!     There is a good description of it in the documentation on the Limetech site.


Parity has no concept of files or data.    It works purely at the physical sector level and as such compression is irrelevant..   Parity works in conjunction with good drives to reconstruct sectors on a failed drive using an appropriate mathematical algorithm.    With a dual parity system the same concept is used with a different algorithm to the first parity     The whole idea is that with a single parity system any single drive failure can be rebuilt using all the other good drives.   With dual parity any two failed drives can be rebuilt.

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Ah okay so the size really is irrelevant only that my Parity drive is bigger or as big as my biggest data drive.

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