db-analyser: Ability to turn off caching for --lsm

[mgmeier]

Description

• ouroboros-consensus-lsm: expose LSM.DiskCachePolicy as a parameter
• db-analyser: allow disabling the disk cache for --lsm via additional flag --lsm-no-cache

Summary

The LSM ledger DB backend previously always used LSM.newTable/LSM.openTableFromSnapshot with their default disk cache policy, so the UTxO table's disk cache behavior was hardcoded and not observable/tunable from outside the library.

This PR:

• Threads a LSM.DiskCachePolicy through LSMArgs, Resources LSM, mkLSMArgsIO, tableFromValuesMK, and loadSnapshot, using LSM.newTableWith/LSM.openTableFromSnapshotWith variants, which allow for overriding the disk cache policy. LSM.DiskCachePolicy is now re-exported from the LSM module.
• Adds a --lsm-no-cache switch to db-analyser, which, when combined with --lsm, sets the policy to LSM.DiskCacheNone (bypassing the OS page cache / using O_DIRECT for UTxO table reads and writes) instead of the default LSM.DiskCacheAll. This has no effect on other backends and is intended primarily for benchmarking disk I/O behavior in isolation from page-cache effects.
• Updates the LSMArgs call sites in the state-machine and ledger-snapshot test suites to pass an explicit cache policy (DiskCacheAll, preserving existing behavior).
• Does not create any code path for propagating some cache policy from a Node config into consensus.

Motivation

For benchmarking the LSM backend's actual disk performance (e.g. comparing read/write throughput or measuring the true cost of table operations), the OS page cache can mask I/O costs by serving repeated reads from memory. Making the disk cache policy configurable lets db-analyser runs isolate on-disk performance from cache effects.

For a full rationale see input-output-hk/ouroboros-consensus-tools#12 and the beacon/docs/METHODOLOGY.md document it adds.

[jasagredo]

Can't this be included in LedgerDBBackend above to only parse it if we are using lsm?

[Russoul]

Addressed 👍