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Improve allocation fallback handling - #17391
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Before this change in case of any allocation error ZFS always fallen back to normal class. But with more of different classes available we migth want more sophisticated logic. For example, it makes sense to fall back from dedup first to special class (if it is allowed to put DDT there) and only then to normal, since in a pool with dedup and special classes populated normal class likely has performance characteristics unsuitable for dedup. This change implements general mechanism where fallback order is controlled by the same spa_preferred_class() as the initial class selection. And as first application it implements the mentioned dedup->special->normal fallbacks. I have more plans for it later. Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored by: iXsystems, Inc.
pcd1193182
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behlendorf
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Status: Accepted
gamanakis
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Before this change in case of any allocation error ZFS always fallen back to normal class. But with more of different classes available we migth want more sophisticated logic. For example, it makes sense to fall back from dedup first to special class (if it is allowed to put DDT there) and only then to normal, since in a pool with dedup and special classes populated normal class likely has performance characteristics unsuitable for dedup. This change implements general mechanism where fallback order is controlled by the same spa_preferred_class() as the initial class selection. And as first application it implements the mentioned dedup->special->normal fallbacks. I have more plans for it later. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Paul Dagnelie <pcd@delphix.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored by: iXsystems, Inc. Closes openzfs#17391
spauka
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Before this change in case of any allocation error ZFS always fallen back to normal class. But with more of different classes available we migth want more sophisticated logic. For example, it makes sense to fall back from dedup first to special class (if it is allowed to put DDT there) and only then to normal, since in a pool with dedup and special classes populated normal class likely has performance characteristics unsuitable for dedup. This change implements general mechanism where fallback order is controlled by the same spa_preferred_class() as the initial class selection. And as first application it implements the mentioned dedup->special->normal fallbacks. I have more plans for it later. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Paul Dagnelie <pcd@delphix.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored by: iXsystems, Inc. Closes openzfs#17391
lundman
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Jan 30, 2026
Before this change in case of any allocation error ZFS always fallen back to normal class. But with more of different classes available we migth want more sophisticated logic. For example, it makes sense to fall back from dedup first to special class (if it is allowed to put DDT there) and only then to normal, since in a pool with dedup and special classes populated normal class likely has performance characteristics unsuitable for dedup. This change implements general mechanism where fallback order is controlled by the same spa_preferred_class() as the initial class selection. And as first application it implements the mentioned dedup->special->normal fallbacks. I have more plans for it later. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Paul Dagnelie <pcd@delphix.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored by: iXsystems, Inc. Closes openzfs#17391
lundman
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Feb 16, 2026
Before this change in case of any allocation error ZFS always fallen back to normal class. But with more of different classes available we migth want more sophisticated logic. For example, it makes sense to fall back from dedup first to special class (if it is allowed to put DDT there) and only then to normal, since in a pool with dedup and special classes populated normal class likely has performance characteristics unsuitable for dedup. This change implements general mechanism where fallback order is controlled by the same spa_preferred_class() as the initial class selection. And as first application it implements the mentioned dedup->special->normal fallbacks. I have more plans for it later. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Paul Dagnelie <pcd@delphix.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored by: iXsystems, Inc. Closes openzfs#17391
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Before this change in case of any allocation error ZFS always fallen back to normal class. But with more of different classes available we migth want more sophisticated logic. For example, it makes sense to fall back from dedup first to special class (if it is allowed to put DDT there) and only then to normal, since in a pool with dedup and special classes populated normal class likely has performance characteristics unsuitable for dedup.
This change implements general mechanism where fallback order is controlled by the same
spa_preferred_class()as the initial class selection. And as first application it implements the mentioned dedup->special->normal fallbacks. I have more plans for it later.While there, I've slightly polished some of dedup quota logic.
How Has This Been Tested?
Created a pool with very small dedup vdev and observed DDT leaking to special vdev. Reduced special vdev size and observed further leaks to normal vdev.
Types of changes
Checklist:
Signed-off-by.