The Cache Repository was introduced in Veeam Backup & Replication v10 and holds a cache of folders information in the structure of the protected NAS share. It is used for NAS backup only and crucial for incremental backup runs. It is also the controlling unit of a NAS backup job.
Whenever a NAS backup job runs, it is the Cache Repository that requests the File Proxy to retrieve data from the source share. The File Proxy sends back the top level structures hashes, the cache repository loads them into memory and initiates a CRC check on the data stored in the cache (generated with the previous job run). If there is a match to the cache value, no file has to be processed and the File Proxy will be instructed to iterate and repeat this process on deeper levels of the share to find folders with a mismatch.
There is one Cache Repository per source share.
Ideally, the Cache Repository should sit as near as possible to the File Proxy and source data: the bigger the share, the more the traffic between File Proxies and Cache Repository. At the best, they should be one hop away from each other.
No disk sizing is required: usually the size of the cache files does not exceed 4 GB and due to the optimized size of the cache, data will be loaded into memory when needed. This also makes fast disks not necessary.
If you have multiple proxy or repository servers, you can distribute the load over all machines. The cache repository is always a standard repository. It cannot be part of a Scale-out Backup Repository, but you can create an extra standard repository on a machine that also hosts Scale-out Backup Repository extents.
As far as compute resources are concerned, we recommend 2/3rds the File Proxy calculated cores and 4GB per core.