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From "Joshua McKenzie (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-8729) Commitlog causes read before write when overwriting
Date Wed, 04 Feb 2015 15:58:35 GMT


Joshua McKenzie commented on CASSANDRA-8729:

While testing for CASSANDRA-6890 (debating removing mmap path), it was pretty clear from my
testing that mmap'ed I/O on Windows has a considerable advantage over buffered to a degree
that linux does not.  I'm of the opinion we should make efforts to memory-map our I/O on Windows
wherever possible, with the known caveat that it makes deleting and renaming files more complicated
(all segments have to be unmapped before either of those ops).

> Commitlog causes read before write when overwriting
> ---------------------------------------------------
>                 Key: CASSANDRA-8729
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Ariel Weisberg
> 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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