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Target ARC size can get reduced to arc_c_min
Sometimes the target ARC size is reduced to arc_c_min, which impacts performance. We've seen this happen as part of the random_reads performance regression test, where the ARC size is reduced before the reads test starts which impacts how long it takes for system to reach good IOPS performance. We call arc_reduce_target_size when arc_reap_cb_check() returns TRUE, and arc_available_memory() is less than arc_c>>arc_shrink_shift. However, arc_available_memory() could easily be low, even when arc_c is low, because we can have tons of unused bufs in the abd kmem cache. This would be especially true just after the DMU requests a bunch of stuff be evicted from the ARC (e.g. due to "zpool export"). To fix this, the ARC should reduce arc_c by the requested amount, not all the way down to arc_size (or arc_c_min), which can be very small. External-issue: DLPX-59431 Signed-off-by: Matthew Ahrens <firstname.lastname@example.org>
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