feat(pageserver): generate image layers for sparse keyspace

[skyzh]

Problem

Part of #7462

Sparse keyspace does not generate image layers for now. This pull request adds support for generating image layers for sparse keyspace.

Summary of changes

• Use the scan interface to generate compaction data for sparse keyspace.
• Track num of delta layers reads during scan.
• Read-trigger compaction: when a scan on the keyspace touches too many delta files, generate an image layer. There are one hard-coded threshold for now: max delta layers we want to touch for a scan.
• L0 compaction does not need to compute holes for metadata keyspace.

Know issue: the scan interface currently reads past the image layer, which causes delta_layer_accessed keeps increasing even if image layers are generated. The pull request to fix that will be separate, and orthogonal to this one.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3102 tests run: 2975 passed, 0 failed, 127 skipped (full report)

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Flaky tests (4)

Postgres 15

• test_location_conf_churn[2]: debug
• test_synthetic_size_while_deleting: debug
• test_vm_bit_clear_on_heap_lock: debug

Postgres 14

• test_compute_pageserver_connection_stress: debug

Code coverage* (full report)

• functions: 31.3% (6396 of 20411 functions)
• lines: 47.9% (48872 of 102126 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
58c801c at 2024-05-20T16:18:29.609Z :recycle:}

[skyzh]

For reference, this is the log of test_metadata_scan, where image layer generation happens and L0 compaction works correctly.

test_metadata_scan.log

[skyzh]

@arpad-m @VladLazar comments resolved and ready for review, thanks :)

[VladLazar]

I'd really like to avoid adding something to the request context that is only needed for compaction. I've got one suggestion here.

[skyzh]

should be ready for review and wait for CI runs...

[VladLazar]

Looks good to me.