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Deduplicate wallog #448
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Deduplicate wallog #448
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**!!! early draft, full of debug prints, barely works !!!** This draft contains experiments around deduplicating our own `wallog` format with libSQL WAL. The potential win here is reducing write and space amplification from 2x to around 1.08x. The main idea is as follows: `wallog` is only used to store frame metadata, and frame data is only stored either in the main database file, or in WAL. That's very simple to implement in a single-node system, but it gets complicated with replicas, because a replica is allowed to ask the primary for any arbitrary wallog frame. The rough idea for dealing with replicas is to: 1. Make sure that we control checkpoints. autocheckpoint is off, and we only issue a checkpoint operation on the primary ourselves, explicitly, and periodically. 2. All streaming of frames to replicas must finish before we issue a checkpoint operation. 3. We only checkpoint in TRUNCATE mode, i.e. a write lock is taken and the whole WAL log is rewritten to the main db file. That simplifies lots of edge (sic!) cases. 4. Once we checkpoint, we drop the previous `wallog`, and instead only store the following information. Let's assume that the main db file has N pages. Pages 1..N are now available as frames X..X+N in the `wallog`, and X is the oldest frame a replica should ever ask for -> anything before X is out-of-date anyway. If any replica asks for an earlier page, it gets an error message saying "please drop whatever you're doing and start asking for frames X or greater instead.
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!!! early draft, full of debug prints, barely works !!!
This draft contains experiments around deduplicating our own
wallogformat with libSQL WAL. The potential win here is reducing write and space amplification from 2x to around 1.08x.The main idea is as follows:
wallogis only used to store frame metadata, and frame data is only stored either in the main database file, or in WAL. That's very simple to implement in a single-node system, but it gets complicated with replicas, because a replica is allowed to ask the primary for any arbitrary wallog frame.The rough idea for dealing with replicas is to:
wallog, and instead only store the following information. Let's assume that the main db file has N pages. Pages 1..N are now available as frames X..X+N in thewallog, and X is the oldest frame a replica should ever ask for -> anything before X is out-of-date anyway. If any replica asks for an earlier page, it gets an error message saying "please drop whatever you're doing and start asking for frames X or greater instead.