Filewalker accesses the metadata of every file. The more metadata the system can keep in RAM cache, the faster filewalker runs. If you have enough RAM to hold the entire metadata, the filewalker runs the fastest. I do not know how to calculate how much metadata is in a particular partition.
The data parition on my node has 29695764 used inodes and one inode is 256B in size, so to cache all of them the VM needs 7.6GB, but there are also directories and probably other stuff that needs to be cached.
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