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Jeremiah Jordan commented on CASSANDRA-8729:
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His fix is actually to make the segment size = commit log size, aka 2 GB, which
means you get one giant segment. That seems a little extreme, and makes commit
log archiving much harder. If this really causes such a big performance
degradation, can we just turn off segment recycle in 2.1? Seems like that
isn't too invasive of a change, since we don't always recycle anyway?
> Commitlog causes read before write when overwriting
> ---------------------------------------------------
>
> Key: CASSANDRA-8729
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8729
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Ariel Weisberg
> Labels: commitlog
> Fix For: 3.0
>
>
> The memory mapped commit log implementation writes directly to the page
> cache. If a page is not in the cache the kernel will read it in even though
> we are going to overwrite.
> The way to avoid this is to write to private memory, and then pad the write
> with 0s at the end so it is page (4k) aligned before writing to a file.
> The commit log would benefit from being refactored into something that looks
> more like a pipeline with incoming requests receiving private memory to write
> in, completed buffers being submitted to a parallelized compression/checksum
> step, followed by submission to another thread for writing to a file that
> preserves the order.
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