I ran into the same issue. Neograph's comment is irrelevant in this case, he's talking about traditional Windows Server disk mirroring, not Storage Spaces. Thin provisioning is also quite irrelevant in this case. You can use it as a workaround, but I think more cautious planning will be better both budget- and performance-wise, just read on and you'll see.
After quite some time spent on reading and playing around in Server Manager, I think I figured out what's going on. The thing is, SS has this thing called "columns". That defines how many disks data is striped across. If your virtual disk was created with 4 columns, data is only spread across 4 disks with Simple layout (i.e. RAID0) or 8 disks with Two-way mirror (i.e. RAID10), not all of them. Now this may be confusing for someone who comes from traditional HW RAID (like me), but that's the way it is.
Note: from now on, I'll refer to the number of columns as column size because it's much more intuitive for me this way.
So anyway, column size also defines how you can extend a virtual disk. Clearly, if your current VD has groups of 4 disks (= column size is 4), you can't add a "half" group by adding 2 new disks. So the number of disks required for expansion is basically
n x NumberOfColumns x NumberOfDataCopies
So if you have a two-way mirror and a column size of 1, you can only add pairs of disks. If your mirror has a column size of 3, you can only add 6, 12, 18 or so disks.
From what I understand, the default column size for a VD is the number of disks divided by copy count, but 8 at maximum, e.g. if you have 10 disks in a two-way mirror, column size will be 5, if you have 16 disks, column size will be 8, but if you have 24 disks, column size will still be 8 - by default. Note: you can check these numbers under VD properties (NumberOfColumns and NumberOfDataCopies properties undes Details).
And here come a lot of headaches:
- the default column size requires you to double the disk count if you want to expand the VD (in most cases)
- you can only select column size if disk usage was set to Manual during pool creation
- the default disk usage is Automatic (of course)
- you cannot change column size once the VD's created
- you cannot change disk usage to Manual once the pool's created
So to have this set up properly, you need to delete:
- the volume
- the virtual disk
- the storage pool
i.e. everything. As a sidenote, stripe size (called Interleave size in SS) is also unavailable if disk usage is set to automatic.
Now you may wonder why would anyone use anything bigger than 1 for column size. The answer is of course performance. The bigger the column size the better the performance you get. Actually, it can be quite dramatic, here's a benchmark with column size 1 and 6:
You need to plan wisely. Only use a high column size if you know for sure that you'll be able to afford to purchase a big number of disks once disk space runs out.
Some good reads on the topic:
Best Answer
During the creation of Storage Pool the Microsoft Storage Spaces using the maximum available size of the disks which can be added to the pool. The same for creating virtual disks in storage pool even without using "Maximum size" parameter. The thing is that your disks do not use exact 50 GB per disk, because:
"To a hard disk manufacturer, one KB is 1000 bytes, one MB is 1000 KB, and one GB is 1000 MB. Essentially, if a hard disk is advertised as 500GB, it contains 500 * 1000 * 1000 * 1000 = 500,000,000,000 bytes of space. The hard disk manufacturer thus advertises the disk as a 500 GB hard disk. However, Microsoft use groups of 1024. When you’re buying memory, a KB is 1024 bytes, a MB is 1024 KB, and a GB is 1024 MB. To work back from the 500,000,000,000 bytes above: 500,000,000,000 / (1024*1024*1024) = 465.66 GB
Keep in mind that the hard drive manufacturers are using the accurate description of the terms–the prefix giga, for instance, means a power of 1000, whereas the correct term for powers of 1024 is gibibyte, though it isn’t often used. Unfortunately, Windows has always calculated hard drives as powers of 1024 while hard drive manufacturers use powers of 1000."