rust: Support writing without chunks #1201
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Topic: nochunk On very memory-constrained systems like microcontrollers, the overhead of storing all the chunk offset data in memory until the end of the file can be significant. To avoid that, we instead write all Channel, Schema, and Message data directly as records rather than using chunks, and rely on the reader (or an intermediate ingestion process) to generate the index information.
HALtheWise
changed the title
eloff
previously requested changes
Jul 24, 2024
Topic: nochunk
eloff
dismissed
their stale review
Issue was resolved, still want @james-rms to have a look before approving this
james-rms
reviewed
Jul 25, 2024
Topic: nochunk
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Done |
eloff
approved these changes
Jul 26, 2024
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On very memory-constrained systems like microcontrollers, the overhead of storing all the chunk offset data in memory until the end of the file can be significant. To avoid that, we instead write all Channel, Schema, and Message data directly as records rather than using chunks, and rely on the reader (or an intermediate ingestion process) to generate the index information.
Changelog
The rust library now (optionally) supports writing unchunked MCAP files for decreased memory footprint.
Docs
None
Description
In our internal testing, storing chunk data required >40KB of memory for several-minute logs, which is a lot for an embedded system with only 300KB of RAM total. Notably, this grows without bound during the test, since the offsets and data about all chunks needs to be stored until the end of the log when it will be written to the index.
For our usecase, it's acceptable to disable chunking entirely (along with compression and indexing), since we can add those things later during the log upload process.
We're using this internally on an ESP32-S3 and it is working well.