Docs: Document prepare.checkpoint_id and constants.checkpoint_interval #1457
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matklad
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Jan 23, 2024
src/constants.zig
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| /// The checkpoint interval must be less than the WAL length by the sum of: | ||
| /// - `lsm_batch_multiple`: Ensure that the final batch of entries immediately preceding a | ||
| /// checkpoint trigger is not overwritten by the following checkpoint's entries. This final | ||
| /// batch's updates were not persisted as part of the former checkpoint – they are only in memory | ||
| /// until they are compacted by the *next* batch of commits (i.e. the first batch of the following | ||
| /// checkpoint). | ||
| /// - `pipeline_prepare_queue_max` (rounded up to the nearest batch multiple): This margin ensures | ||
| /// that the entries prepared immediately following a checkpoint trigger never overwrite an entry | ||
| /// from the previous WAL wrap until a quorum of replicas has reached that checkpoint. | ||
| pub const vsr_checkpoint_interval = journal_slot_count - | ||
| lsm_batch_multiple - | ||
| lsm_batch_multiple * stdx.div_ceil(pipeline_prepare_queue_max, lsm_batch_multiple); |
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Let's maybe move the two points down to assert bellow, and here instead say that we pick the largest checkpoint interval that satisfies the assert.
Basically, while the assert below and the initialization here looks redundant, they are not: we can pick any value between lsm_batch_multiple and the upper bound, it's just that picking exactly the upper bound is most optimal for performance.
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matklad
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