Why Virtual Disk has 2 Column Layout on S2D Setup?

Question

Hi there

I would like to install the smallest possible S2D Setup on two VMs only for high availability file sharing services. I think this would be interesting also for small companies.

So i set up two VMs with tree disks (one OS and two for S2D) each. When creating Volumes using the New-Volume cmdlet or when creating disks manually, there are always in a 2-column / mirrored layout. Therefore all data is mirrored across the two clustered VMs (which is fine) and all data are written to both disks virtual disks on each vm additionally.

Is it supported from the S2D perspective to create virtual disks with a mirrored 1-column layout? This would save a lot of space down on the hyper-v host. I don't see the sense of having two vhdx per VM while both of the vhdx files sitting on the same SSD on host (and therefore consumes the double amount of space).

Any thoughts?

Many thanks

Thread source link: https://social.technet.microsoft.com/Forums/windowsserver/en-US/641e0c68-c6e7-4bed-ab7d-d7382ec40c19/why-virtual-disk-has-2-column-layout-on-s2d-setup?forum=winserverClustering

Answer 1

Hi Paraplexo,

If i now configure the virtual disk on the VMs manually with a 1 column but still mirrored, this should reduce the footprint, right? I tested already, this seems to work.

What do you mean "it works", we can change the column, but does the total data size reduce, in your example, two-way mirror 2 column of 400G, and two-way mirror 1 column decrease to 200G?

As far as I'm concerned, it won't. It should be two-way mirror 2 column 200G and two-way mirror 1 column 200G. The data in total is depended on the mirror type, not the column.

For two-way mirror 1 column with 4 disks, d1, d2, d3, d4; for example, d1+d2 is the primary disk, d3,d4 is the mirror disk; when it writes data, it will write default primary 256k stripe to d1, mirrored 256k to d3, when d1 is full, write primary 256k stripe to d2, mirrored 256K to d4;

For two-way mirror 2 column with 4 disks, d1,d2,d3,d4; d1+d3 in column1, d2+d4 in column2, when write data, one primary 256K into d1, mirror 256K into d3, the following 256K into d2, mirrored 256K into d4. In this way, the following 256K has no need to wait for the above 256 finish writing into d1, it can be quickly written into d2.

So two-way mirror 2 column with 4 disks will increase the performance.

Related article for your reference:

https://social.technet.microsoft.com/wiki/contents/articles/11382.storage-spaces-frequently-asked-questions-faq.aspx#Example_3_A_Two-Column_Two-Way_Mirror_Space

Best Regards,

Anne