I try to add three SSDs as journal disks to an existing Storage Space (with powershell commands Add-PhysicalDisk and Set-PhysicalDisk -usage journal).
However strangely this increased the size of the storage pool (get-storagepool | select size).
How can I detect whether the added disks are really used for Parity Journal and Write Back Cache? Or is it not possible to add this to an already existing storage space?
Get-PhysicalDisk PowerShell cmdlet is your best friend. Loop over all disks in your system checking "-Usage" parameter. For journal disks it will be "Journal" (surprise!). If there are no "Journal" disks detected your pool isn't using journalling disks.
https://docs.microsoft.com/en-us/powershell/module/storage/get-physicaldisk?view=win10-ps
I use this to verify Journal on my parity space for my F: drive (serialnumber / uniqueid omitted here, only 26 GB at the end of the the SSD are used for the Journal, the rest is a normal drive):
PS C:\> Get-StoragePool -FriendlyName Pool-F | Get-PhysicalDisk | ft FriendlyName,MediaType,HealthStatus,canpool,CannotPoolReason,size,uniqueid,SerialNumber,Usage FriendlyName MediaType HealthStatus canpool CannotPoolReason size Usage ------------ --------- ------------ ------- ---------------- ---- ----- WDC WD30EFRX-68AX9N0 HDD Healthy False In a Pool 3000592982016 Auto-Select WDC WD30EFRX-68AX9N0 HDD Healthy False In a Pool 3000592982016 Auto-Select Samsung SSD 860 EVO 4TB SSD Healthy False In a Pool 4000787030016 Journal ST33000651AS HDD Healthy False In a Pool 3000592982016 Auto-Select 