pageserver: decrease utilization MAX_SHARDS

[jcsp]

Problem

The intent of this parameter is to have pageservers consider themselves "full" if they've got lots of shards, even if they have plenty of capacity. It works, but because we typically successfully oversubscribe capacity up to 200%, the MAX_SHARDS limit is effectively doubled, so this 20,000 value ends up meaning 40,000, whereas the original intent was to limit nodes to ~10000 shards.

Summary of changes

• Change MAX_SHARDS to 5000, so that a node with 5000 will get a 100% utilization, which is equivalent in practice to being considered "half full" by the storage controller in capacity terms.

This is all a bit subtle and indiret. Originally the limit was baked into the pageserver with the idea that the pageserver knows better what its own resources tolerate than the storage controller does, but in practice it would be probably be easier to understand all this if we just did it controller-side. So there's scope to refactor here in future.

[github-actions]

7403 tests run: 7017 passed, 0 failed, 386 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_pageserver_gc_compaction_smoke[with_branches]: release-x86-64-without-lfc

Postgres 14

• test_replica_query_race: release-x86-64-with-lfc
• test_pageserver_restart: release-arm64-with-lfc

Code coverage* (full report)

• functions: 33.5% (8495 of 25347 functions)
• lines: 49.3% (71482 of 145075 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
a746fb1 at 2025-01-27T17:11:27.013Z :recycle:}