I'm working on a new nas to store veeam backups of some VMs.
I will use some magnetic disks and my doubt is about optional ssd cache disks.
I was wondering if it is important the cache layer, generally I know few cache disks is not a bad idea, but in this case I would like to know if it is recommended or not very useful.
As far as I know the cache is used to absorb few small operations or repeated operations in same sectors. In a vm backup scenario I expect a massive write ops and cache layer will be shortly filled and became almost unuseful.
Am I right or am I wrong?
It depends.
During backup, large contiguous files are written and a flash cache isn't of much use. It might be able to optimize concurrently running jobs - depending on the caching mode and algorithms, but the volume of writes is going to wear down a normal SSD pretty quickly.
For VM recovery to production storage, there isn't much benefit either. However, if you plan to use Veeam Instant Recovery, especially without immediate migration, then a reasonably sized flash cache can make quite a difference.