Implement allocation size ranges and use for gang leaves #17111
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When forced to resort to ganging, ZFS currently allocates three child blocks, each one third of the size of the original. This is true regardless of whether larger allocations could be made, which would allow us to have fewer gang leaves. This improves performance when fragmentation is high enough to require ganging, but not so high that all the free ranges are only just big enough to hold a third of the recordsize. This is also useful for improving the behavior of a future change to allow larger gang headers. We add the ability for the allocation codepath to allocate a range of sizes instead of a single fixed size. We then use this to pre-allocate the DVAs for the gang children. If those allocations fail, we fall back to the normal write path, which will likely re-gang. Signed-off-by: Paul Dagnelie <[email protected]>
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Status: Accepted
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Motivation and Context
This PR pulls out the dynamic allocation size changes from #17004 into their own PR. It also removes the de-ganging logic, which isn't necessary and is difficult to implement without some pretty unpleasant-looking code.
Description
We teach the metaslab code how to allocate a range of sizes, not just a single target size, and use that in the gang block code to try to get it to allocate fewer leaves when possible.
How Has This Been Tested?
Manual testing + the new gang block test.
Types of changes
Checklist:
Signed-off-by.