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We have two servers that we are attaching SAS expansion bays to expand our storage, but one server is running Windows Server 2012 and the other is running Windows Server 2012 R2. We are using a SAS HBA and planning on setting up identical storage spaces on each server.

On Windows Server 2012 R2 we are setting up a virtual disk with dual parity and fixed provisioning at 100TB. The total space used by this disk is about 121.5TB

On Windows Server 2012 we are setting up (as close as we can) the same thing. It is a virtual disk with parity selected (dual parity is not an explicit option) fixed provisioning at 100TB. The total space used by this disk is about 114.3TB

I'm confused because according to Microsoft here Storage Spaces FAQ they suggest that if you have more than 7 drives (which we do) then you should be protected from two drive failures (reading as dual parity or RAID6)

Parity spaces are designed for capacity efficiency and increased resiliency. Parity spaces are best suited for archival data and streaming media, such as music and videos. This storage layout requires at least three disks to protect you from a single disk failure and at least seven disks to protect you from two disk failures.

Questions:

Are we actually protected from two drive failures in both configurations?

If we aren't protected from two drive failures in a parity configuration on Server 2012, then is there any way to get around that other than doing a mirror or upgrading to Server 2012 R2?

If we are protected from two drive failures in both configurations, then why the large difference in amount of drive space used by the different configurations?

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Dual Parity is a feature of storage spaces in 2012 R2, this TechNet link describes the new features of storage spaces introduced with 2012R2 Dual Parity being one of them.

https://technet.microsoft.com/en-us/library/dn387076.aspx#bkmk_dualparity

Upgrading your 2012 server to 2012 R2 will make this feature available plus Storage Tiers and Write Back caching (both excellent features).

with just Server 2012 you are only protected from a single disk failure.

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