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On a system with very high fragmentation, we may need to do lots of gang allocations (e.g. most indirect block allocations (~50KB) may need to gang). Before failing a "normal" allocation and resorting to ganging, we try every metaslab. This has the impact of loading every metaslab (not a huge deal since we now typically keep all metaslabs loaded), and also iterating over every metaslab for every failing allocation. If there are many metaslabs (more than the typical ~200, e.g. due to vdev expansion or very large vdevs), the CPU cost of this iteration can be very impactful. This iteration is done with the mg_lock held, creating long hold times and high lock contention for concurrent allocations, ultimately causing long txg sync times and poor application performance. To address this, this commit changes the behavior of "normal" (not try_hard, not ZIL) allocations. These will now only examine the 100 best metaslabs (as determined by their ms_weight). If none of these have a large enough free segment, then the allocation will fail and we'll fall back on ganging. To accomplish this, we will now (normally) gang before doing a `try_hard` allocation. Non-try_hard allocations will only examine the 100 best metaslabs of each vdev. In summary, we will first try normal allocation. If that fails then we will do a gang allocation. If that fails then we will do a "try hard" gang allocation. If that fails then we will have a multi-layer gang block. Reviewed-by: Paul Dagnelie <pcd@delphix.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Matthew Ahrens <mahrens@delphix.com> Closes #11327 |
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