ADS Cluster – Allocation Unit Size
The general thinking is that a 64K cluster size would be a good choice if the server is targeted mainly as a database server and if the drive is a traditional hard drive (spinning rather than an SSD). The reduced physical fragmentation would provide some valuable performance gains. But there are a couple of things to consider:
- First is the ISAM table format that Advantage uses.
- Records are contiguous in a table and do not correspond to any cluster size; it is purely based on record size.
- It is possible that certain operations (individual record updates in particular) might have better performance with a small cluster size.
- An operation that reads and then updates one single record might be faster if the record resides on a 4K cluster as opposed to a 64K cluster.
- With a physical spinning disk, the bulk of the cost is latency, but it would still take a bit longer to physically read/write a 64K cluster vs a 4K cluster for a single record update.
Index files are another consideration. The page size for indexes is relatively small (they can be up to 4K for the Advantage proprietary table format). If the index is logically fragmented (index pages that are logically adjacent are not physically adjacent on disk), then reading one page after another might be faster with a small cluster size. A re-index operation, though, can fix this and put pages back in logical order.