Storage Spaces Direct Mirroring vs MRV (Parity) performance

HP Lefthand

Back several years ago (about 6 or 7 if i remember correctly) when Storage Spaces and Storage Spaces Direct (S2D) did not exist yet, there was a another vendor called “Lefthand” which did kind of the same trick. Lefthand was bought by HP and the product was renamed to P4000 and later on HP StorVirtual 4000. The principal of this type of storage is different to other vendors. These storage nodes use local raid level on several disks in a system and additional nodes with the exact same hardware are pooled into a cluster. On top of the cluster volumes with network raid level’s are created. So disks in a system (1 disk in case of a single local RAID 5 pool of disks) could die without losing the node. In this setup you have several layers of redundancy on a storage node, but also on the entire cluster.

You could start with 2 systems and create a mirrored volume. The raw capacity of 1 node minus the raid level was the total usable capacity. So take a two node setup with 12x 1TB disks you have give or take 21TB of usable capacity (12 disks in a RAID 5 = 12TB minus 1TB and minus some lost bits and bytes so give or take 10,5TB usable). In a two node setup you will have 10,5 TB of usable space with mirrored volumes because the data is mirrored across both nodes. Mirroring data like that brings you high availability on storage on a node level. So you could loss a storage node without the volume going offline but it will cost you half of the raw storage capacity. If you add an extra node to make a total of 3 nodes, you would have 10,5 * 3 = 31,5TB of raw capacity. Taking the mirroring in consideration you will have about 15,7TB of usable capacity. And this keeps going in a 10 node setup you have 105TB of raw capacity and about 50+TB of usable capacity. So all the time you will loss halve of the capacity in a Network Mirror volume. If you chose for 3-Way or even a 4-Way mirror (don’t know why but it is possible) you have massive redundancy and performance but a terrible efficiency because in a 4-Way mirror on 4 nodes you only have 25% of the RAW capacity available for data.

Yes.. it’s seems like a waste of space, so Lefthand (and later on HP) came up with Network RAID 5. When you have three or more nodes you could setup a volume with Network RAID 5. Then the data is places on 2 nodes and the 3 node is doing parity. You could still lose a disk in a node or an entire node…. BUT it was dreadfully slow and HP recommended AGAINST setting up Network RAID Level 5….
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So I hear you thinking what is with all the “old stuff” on Lefthand… Well, Storage Spaces Direct is kind of the same principal and the same applies on Parity volumes… But bear with me on this 🙂
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Mirror vs MRV Volumes.

With Storage Spaces the resiliency level is set on Volume level. That means that you can create Mirrored and Parity volumes and also a new flavor named Mixed Resiliency Volumes with both Parity and Mirrored space. With traditional hardware RAID, mirrored disks are always faster as Parity disks because of the parity process. The same applies for S2D. With Mirrored volumes all data is mirrored across an x amount of nodes and disks in the cluster. By default Storage Space Direct uses a 3-way mirror layout. All blocks written to disk are copied to 2 other nodes (in case of a 3 node or higher cluster). Because of this you default lose 2/3 of your raw capacity.

Microsoft S2D Program Manager Cosmos Darwin created a nice website to make some calculations on how much usable space you get with different combinations of disks, capacity and resiliency settings, check it out on http://aka.ms/s2dcalc

When you create a 1 TB 3-way mirrored volume that volume has a 3 TB footprint. That’s a simple calculation because 1 TB of data is copied 2 additional times in a 3 way-Mirror which makes 3 TB. When you create an MRV of 1 TB with for example 30% Mirrored capacity and 70% Parity capacity we have to do a bit more math. So 300 GB * 3 is 900 GB. Then we have 700 GB parity space that will require double the space for that a total of 1400 GB. The total footprint of a 1 TB MRV Disk is 900 GB + 1400 GB = 2300 GB. So with an MRV disk you save 700 GB of space on a 1 TB volume.

Because of the massive loss of capacity with 3-way mirror people (most of them are the people who are responsible for the budget) are forcing or highly recommending to use or consider Parity or a form of Mixed reciliency to get more GBs/TB’s out of there hardware.. But at what cost?

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