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From "Brian O'Neill (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-14466) Enable Direct I/O
Date Thu, 07 Jun 2018 15:07:00 GMT


Brian O'Neill commented on CASSANDRA-14466:

Although I don't know how Cassandra behaves in this case, my own experiments with O_DIRECT
have show that it only improves performance in very few applications. In particular, applications
which do very little computation (low CPU load) and are accessing only the fastest available
SSDs. Cassandra tends to be a bit heavy on the CPU load, and so I'm skeptical that switching
to O_DIRECT by itself is the reason performance improved. In addition, I've only seen significant
improvement with O_DIRECT when bypassing the file system and accessing the block device directly.

As was already suggested, switching off read-ahead might be the reason why you're seeing improved
performance. Although the file system is generally able to adapt to usage patterns, I've found
that explicitly setting POSIX_FADV_RANDOM really helps for random access workloads. This behavior
is implicit with O_DIRECT. Although I see that Cassandra has utility code to call fadvise,
only the POSIX_FADV_DONTNEED option ever appears to be used.

Considering that the test machine had 128GB of RAM, you really want the OS to manage the cache
instead of Cassandra. How large was the JVM heap when running the test? Far larger data sizes,
caching more data in the Java heap will lead to GC problems.

> Enable Direct I/O 
> ------------------
>                 Key: CASSANDRA-14466
>                 URL:
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Local Write-Read Paths
>            Reporter: Mulugeta Mammo
>            Priority: Major
>         Attachments: direct_io.patch
> Hi,
> JDK 10 introduced a new API for Direct IO that enables applications to bypass the file
system cache and potentially improve performance. Details of this feature can be found at
> This patch uses the JDK 10 API to enable Direct IO for the Cassandra read path. By default,
we have disabled this feature; but it can be enabled using a  new configuration parameter,
enable_direct_io_for_read_path. We have conducted a Cassandra read-only stress test and measured
a throughput gain of up to 60% on flash drives.
> The patch requires JDK 10 Cassandra Support - 
> Please review the patch and let us know your feedback.
> Thanks,
> [^direct_io.patch]

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