Create new virtual disks in Server 2012 R2 with writeback cache

On a new server I want to create a new virtual disk with writeback cache. The server has 3 physical disks, which is visible for the operating system as one, because it is in hardware-raid5 available. The operating system uses 140Gb of this disk. The server has a 120Gb SSD (unused). I now want to configure the rest of the available space of the harddisk for some partitions, using the SSD as writeback cache. To avoid, that just 1Gb is used as writeback cache, I have to use Powershell. So I have created a Storagepool, and dropped what is available in it. Then I used Powershel to create a SSDtier and HDDtier. As far as I can find, there is no option to specify sizes.  The results are stored in $SSDtier and $HDDtier. Then I use Powershell to create a new virtiual disk. This is no success. After some trials with the syntax, the error now is: there are not enough eligible physical disks in the storage pool. The command I used is:
 new-virtualdisk -StoragePoolFriendlyName "Pool1" -Friendlyname "Samen" -StorageTiers $SSDtier, $HDDtier -Storagetiersizes 10GB, 200Gb -Resiliencysettingname Simple -Writecachesize 50Gb
It would be better, if the 10Gb was 0Gb,  but just to exclude this is part of the problem, I used these values.
The free size on SSDis 115G, and on the HDD is 490G
So what is wrong with this command?
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Cliff GaliherCommented:
RAID controllers and storage spaces don't get along. To do what you want, you can't use RAID. Storage spaces must see the actual physical disks, not just a logical disk presented by the controller.
ebrinkAuthor Commented:
Forget about Raid5. (I am sure, Windows doesn't know this). Read the case as following: The server has 1 disk of 557G (bus type RAID)  and one disk of 115G (bus type SATA). The first Media type is Unkown (it is HDD), the latter is SSD.
Cliff GaliherCommented:
"bus type RAID". That's still the problem.  Storage spaces needs to see the disk(s) full stop. That means using an HBA instead of a RAID controller or using a RAID controller that supports sone sort of JBOD mode. The bus type needs to be SATA, SAS, etc. RAID will fail.
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ebrinkAuthor Commented:
To be failsafe it is required, to have the disks in Raid5 in hardware. Having done this, what is the best option to use the remaining diskspace for data, and caching it with the unused SSD?
Cliff GaliherCommented:
Can't do it with storage spaces. You can get redundancy with storage spaces without RAID. Or you can buy a RAID controller that supports using am SSD as a cache. But you can't use RAID and storage spaces together. You'll have toymaker a choice.
ebrinkAuthor Commented:
In order to check what is possible, I used the GUI to create virtual disks. Some strange behaviour: I lets me create a virtual disk without tiering of 218Gb, with 1Gb cache, where the free size of the pool is 530Gb. Then I can create a second virtual disk, of 310Gb, with no cache.  Finally the pool has 768Mb free space. What happened?  What is the reason, that  I can't use the full capacity in one disk? And why is the first oportunity to create a vdisk a smaller size than the second?
Cliff GaliherCommented:
As long as you insist on using RAID, you'll get unpredictable results. This is known, documented, and is totally unsupported.
ebrinkAuthor Commented:
Analysing what happened, the conclusion is: In this situation, You can create a virtual disk using the SSD combined with the same portion of the HDD (bus type RAID, and then the rest of the HDD for another disk. The difference is: The first vdisk has 2 collumns, the latter has one.
 To verify this, I used the following command:
 new-virtualdisk -StoragePoolFriendlyName "Pool1" -Friendlyname "Samen"  -size 110Gb -Resiliencysettingname Simple -Writecachesize 100gb
This gives me a virtual disk of 110Gb with a cache of 100Gb.
No tiering in this case.
I assume, the cache is on the SSD. No idea how I can verify this.

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ebrinkAuthor Commented:
Using the GUI, I created an extra Vdisk containing the rest of the space. It gets 1Gb of cache.
Deleting this Vdisk, and using powershell, I finally issued the next comand:
new-virtualdisk -StoragePoolFriendlyName "Pool1" -Friendlyname "Samen"  -size 308Gb -Resiliencysettingname Simple -Writecachesize 11gb
This also executes without errors.
And gives me a second  cached disk, eating all space of the SSD for cache.
This should be a safe configuration (I don't trust SSDs).
Note, that the collumns are now for both vdisks 1.
ebrinkAuthor Commented:
The Raid5 controller completely hides the physical disks, and presents the result as a disk. So Microsoft can not do things wrong to corrupt the Raid5 structure. That is why is is no problem to put it in a storage pool, and combine it with an SSD to cache is.
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Windows Server 2012

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