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compress metadata in later sync passes
Starting in sync pass 5 (zfs_sync_pass_dont_compress), we disable compression (including of metadata). Ostensibly this helps the sync passes to converge (i.e. for a sync pass to not need to allocate anything because it is 100% overwrites). However, in practice it increases the average number of sync passes, because when we turn compression off, a lot of block's size will change and thus we have to re-allocate (not overwrite) them. It also increases the number of 128KB allocations (e.g. for indirect blocks and spacemaps) because these will not be compressed. The 128K allocations are especially detrimental to performance on highly fragmented systems, which may have very few free segments of this size, and may need to load new metaslabs to satisfy 128K allocations. We should increase zfs_sync_pass_dont_compress. In practice on a highly fragmented system we see a few 5-pass txg's, a tiny number of 6-pass txg's, and no txg's with more than 6 passes. Reviewed-by: Brian Behlendorf <firstname.lastname@example.org> Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com> Reviewed by: Pavel Zakharov <email@example.com> Reviewed-by: Serapheim Dimitropoulos <firstname.lastname@example.org> Reviewed-by: George Wilson <email@example.com> Signed-off-by: Matthew Ahrens <firstname.lastname@example.org> External-issue: DLPX-63431 Closes #8892
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