Fix metaslab group fragmentation math #17037
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pcd1193182
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Feb 7, 2025
Since we are calculating a free space fragmentation, we should weight metaslabs by the amount of their free space, not a full size. Fragmentation of full metaslabs may not matter in presence empty ones. The old algorithm did not differentiate metaslabs having only one free 4KB block from metaslabs having 50% of space free in 4KB blocks, reporting higher fragmentation. While there, move metaslab_group_alloc_update() call after setting mg_fragmentation, otherwise the effect may be delayed by one TXG. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
tonynguien
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Feb 10, 2025
amotin
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Status: Accepted
tonyhutter
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Feb 18, 2025
ixhamza
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Feb 19, 2025
Since we are calculating a free space fragmentation, we should weight metaslabs by the amount of their free space, not a full size. Fragmentation of full metaslabs may not matter in presence empty ones. The old algorithm did not differentiate metaslabs having only one free 4KB block from metaslabs having 50% of space free in 4KB blocks, reporting higher fragmentation. While there, move metaslab_group_alloc_update() call after setting mg_fragmentation, otherwise the effect may be delayed by one TXG. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Reviewed-by: Paul Dagnelie <[email protected]> Reviewed-by: Tony Nguyen <[email protected]> Reviewed-by: Tony Hutter <[email protected]>
ixhamza
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Feb 25, 2025
Since we are calculating a free space fragmentation, we should weight metaslabs by the amount of their free space, not a full size. Fragmentation of full metaslabs may not matter in presence empty ones. The old algorithm did not differentiate metaslabs having only one free 4KB block from metaslabs having 50% of space free in 4KB blocks, reporting higher fragmentation. While there, move metaslab_group_alloc_update() call after setting mg_fragmentation, otherwise the effect may be delayed by one TXG. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Reviewed-by: Paul Dagnelie <[email protected]> Reviewed-by: Tony Nguyen <[email protected]> Reviewed-by: Tony Hutter <[email protected]>
ixhamza
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Feb 25, 2025
Since we are calculating a free space fragmentation, we should weight metaslabs by the amount of their free space, not a full size. Fragmentation of full metaslabs may not matter in presence empty ones. The old algorithm did not differentiate metaslabs having only one free 4KB block from metaslabs having 50% of space free in 4KB blocks, reporting higher fragmentation. While there, move metaslab_group_alloc_update() call after setting mg_fragmentation, otherwise the effect may be delayed by one TXG. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Reviewed-by: Paul Dagnelie <[email protected]> Reviewed-by: Tony Nguyen <[email protected]> Reviewed-by: Tony Hutter <[email protected]>
ixhamza
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Feb 27, 2025
Since we are calculating a free space fragmentation, we should weight metaslabs by the amount of their free space, not a full size. Fragmentation of full metaslabs may not matter in presence empty ones. The old algorithm did not differentiate metaslabs having only one free 4KB block from metaslabs having 50% of space free in 4KB blocks, reporting higher fragmentation. While there, move metaslab_group_alloc_update() call after setting mg_fragmentation, otherwise the effect may be delayed by one TXG. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Reviewed-by: Paul Dagnelie <[email protected]> Reviewed-by: Tony Nguyen <[email protected]> Reviewed-by: Tony Hutter <[email protected]>
lundman
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Jun 2, 2025
Since we are calculating a free space fragmentation, we should weight metaslabs by the amount of their free space, not a full size. Fragmentation of full metaslabs may not matter in presence empty ones. The old algorithm did not differentiate metaslabs having only one free 4KB block from metaslabs having 50% of space free in 4KB blocks, reporting higher fragmentation. While there, move metaslab_group_alloc_update() call after setting mg_fragmentation, otherwise the effect may be delayed by one TXG. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Reviewed-by: Paul Dagnelie <[email protected]> Reviewed-by: Tony Nguyen <[email protected]> Reviewed-by: Tony Hutter <[email protected]>
lundman
pushed a commit
to openzfsonosx/openzfs-fork
that referenced
this pull request
Jun 2, 2025
Since we are calculating a free space fragmentation, we should weight metaslabs by the amount of their free space, not a full size. Fragmentation of full metaslabs may not matter in presence empty ones. The old algorithm did not differentiate metaslabs having only one free 4KB block from metaslabs having 50% of space free in 4KB blocks, reporting higher fragmentation. While there, move metaslab_group_alloc_update() call after setting mg_fragmentation, otherwise the effect may be delayed by one TXG. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Reviewed-by: Paul Dagnelie <[email protected]> Reviewed-by: Tony Nguyen <[email protected]> Reviewed-by: Tony Hutter <[email protected]>
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Since we are calculating a free space fragmentation, we should weight metaslabs by the amount of their free space, not a full size. Fragmentation of full metaslabs may not matter in presence empty ones. The old algorithm did not differentiate metaslabs having only one free 4KB block from metaslabs having 50% of space free in 4KB blocks, reporting higher fragmentation.
While there, move metaslab_group_alloc_update() call after setting mg_fragmentation, otherwise the effect may be delayed by one TXG.
How Has This Been Tested?
Sequentially filled half of metaslabs to almost full, leaving half almost empty, and observed incorrect 8% fragmentation. Applied the patch and saw the proper 0%. Randomly overwritten the data and observed that fragmentation shown actual 10% due to plenty of small holes.
Types of changes
Checklist:
Signed-off-by.