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The ARC caches data in scatter ABD's, which are collections of pages, which are typically 4K. Therefore, the space used to cache each block is rounded up to a multiple of 4K. The ABD subsystem tracks this wasted memory in the `scatter_chunk_waste` kstat. However, the ARC's `size` is not aware of the memory used by this round-up, it only accounts for the size that it requested from the ABD subsystem. Therefore, the ARC is effectively using more memory than it is aware of, due to the `scatter_chunk_waste`. This impacts observability, e.g. `arcstat` will show that the ARC is using less memory than it effectively is. It also impacts how the ARC responds to memory pressure. As the amount of `scatter_chunk_waste` changes, it appears to the ARC as memory pressure, so it needs to resize `arc_c`. If the sector size (`1<<ashift`) is the same as the page size (or larger), there won't be any waste. If the (compressed) block size is relatively large compared to the page size, the amount of `scatter_chunk_waste` will be small, so the problematic effects are minimal. However, if using 512B sectors (`ashift=9`), and the (compressed) block size is small (e.g. `compression=on` with the default `volblocksize=8k` or a decreased `recordsize`), the amount of `scatter_chunk_waste` can be very large. On a production system, with `arc_size` at a constant 50% of memory, `scatter_chunk_waste` has been been observed to be 10-30% of memory. This commit adds `scatter_chunk_waste` to `arc_size`, and adds a new `waste` field to `arcstat`. As a result, the ARC's memory usage is more observable, and `arc_c` does not need to be adjusted as frequently. Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: George Wilson <gwilson@delphix.com> Reviewed-by: Ryan Moeller <ryan@iXsystems.com> Signed-off-by: Matthew Ahrens <mahrens@delphix.com> Closes #10701 |
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