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With compressed ARC (bug 6950) we use up to 25% of our CPU to decompress indirect blocks, under a workload of random cached reads. To reduce this decompression cost, we would like to increase the size of the dbuf cache so that more indirect blocks can be stored uncompressed. If we are caching entire large files of recordsize=8K, the indirect blocks use 1/64th as much memory as the data blocks (assuming they have the same compression ratio). We suggest making the dbuf cache be 1/32nd of all memory, so that in this scenario we should be able to keep all the indirect blocks decompressed in the dbuf cache. (We want it to be more than the 1/64th that the indirect blocks would use because we need to cache other stuff in the dbuf cache as well.) In real world workloads, this won't help as dramatically as the example above, but we think it's still worth it because the risk of decreasing performance is low. The potential negative performance impact is that we will be slightly reducing the size of the ARC (by ~3%). Porting Notes: * Added modules options to zfs-module-parameters.5 man page. * Preserved scaling based on target ARC size rather than max ARC size. Authored by: George Wilson <george.wilson@delphix.com> Reviewed by: Dan Kimmel <dan.kimmel@delphix.com> Reviewed by: Prashanth Sreenivasa <pks@delphix.com> Reviewed by: Paul Dagnelie <pcd@delphix.com> Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com> Reviewed-by: George Melikov <mail@gmelikov.ru> Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov> Ported-by: Brian Behlendorf <behlendorf1@llnl.gov> OpenZFS-issue: https://www.illumos.org/issues/9188 OpenZFS-commit: https://github.com/openzfs/openzfs/pull/564 Upstream bug: DLPX-46942 Closes #7273 |
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