I’m copying a large a mount of data from the TrueNAS to another NAS. TrueNAS has a SSD based cache and the other NAS is completely SSD based. The is used but only written to and not read. I wonder why that is?
There is no SSD-based cache in the strict sense of the word in ZFS. Without a lot more detail about your hardware and which steps you did during testing no help is possible
“cache” here is L2ARC. Why would L2ARC be read while “copying a large amount of data”, that is a one-off operation, not reading the same data twice?
I expected, probably wrongly, that the l2arc is used for consequetive data to copied to and than used as a faster access. But indeed the data is used only once so in this case the l2arc space is just wasted on data that is used only once.
See the screen shot it is a raidz1 set with a cache (l2arc) ssd deviced attached. What other detail would be helpfull?
Wrong expectation indeed. L2ARC is filled with data that is evicted from ARC, not filled preemptively.
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