Better fill empty metaslabs #17081
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Better fill empty metaslabs #17081
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Before this change zfs_metaslab_switch_threshold tunable switched metaslabs each time ones index reduced by two (which means biggest contiguous chunk reduced to 1/4). It is a good idea to balance metaslabs fragmentation. But for empty metaslabs (having power- of-2 sizes) this means switching when they get just below the half of their capacity. Inspection with zdb after filling new pool to half capacity shown most of its metaslabs filled to half capacity. I consider this sub-optimal for pool fragmentation in a long run. This change blocks the metaslabs switching if most of the metaslab free space (15/16) is represented by a single contiguous range. Such metaslab should not be considered fragmented until it actually fail some big allocation. More contiguous filling should improve data locality and increase time before previously filled and partially freed metaslab is touched again, giving it more time to free more contiguous chunks for lower fragmentation. It should also slightly reduce spacemap traffic. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
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Motivation and Context
Before this change zfs_metaslab_switch_threshold tunable switched metaslabs each time ones index reduced by two (which means biggest contiguous chunk reduced to 1/4). It is a good idea to balance metaslabs fragmentation. But for empty metaslabs (having power-of-2 sizes) this means switching when they get just below the half of their capacity. Inspection with zdb after filling new pool to half capacity shown most of its metaslabs filled to half capacity. I consider this sub-optimal for pool fragmentation in a long run.
Description
This change blocks the metaslabs switching if most of the metaslab free space (15/16) is represented by a single contiguous range. Such metaslab should not be considered fragmented until it actually fail some big allocation. More contiguous filling should improve data locality and increase time before previously filled and partially freed metaslab is touched again, giving it more time to free more contiguous chunks for lower fragmentation. It should also slightly reduce spacemap traffic.
How Has This Been Tested?
Filled an empty pool to half capacity, observed with zdb that all metaslabs were ~50% full. After applying the patch and repeating, observed half of metaslabs filled to ~100% and the rest are almost empty.
This change actually made me to create #17037, since old math was reporting the pool written in a new way as badly fragmented.
Types of changes
Checklist:
Signed-off-by.