I'm wondering if it's possible somehow create RAID0 which consist of RAM disk and more regular disks. I need improve performance of that RAID0 as much as I can but RAM disk is too small for my purposes and RAM chache for RAID0 is useless because it contains always different data.
No I am afraid a RAM disk can't be used in RAID array. But even if it were possible, it would make little sense.
While RAID0 can be created with disks of differing sizes, but the storage space added to the array by each disk is limited to the size of the smallest disk. For example, if a 16 GB RAM disk is striped together with a 100 GB SDD, the size of the array will be 32 GB (16 GB × 2).
Another issue with this would be that if the contents of the RAM disk is lost, i.e. through a corrupted image file or simple reboot or power loss if images are not used, it would mean the loss of all data in the array.