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https://issues.apache.org/jira/browse/CASSANDRA-1241?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Robert Coli updated CASSANDRA-1241:
-----------------------------------
Affects Version/s: (was: 0.3)
(was: 0.4)
(was: 0.5)
(was: 0.6.4)
> config file option DiskAccessMode has no-op option "mmap_index_only"
> --------------------------------------------------------------------
>
> Key: CASSANDRA-1241
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1241
> Project: Cassandra
> Issue Type: Bug
> Components: Core, Documentation & website
> Affects Versions: 0.6, 0.6.1, 0.6.2, 0.6.3, 0.7
> Reporter: Robert Coli
>
> Per http://wiki.apache.org/cassandra/StorageConfiguration :
> "
> Access mode. mmapped i/o is substantially faster, but only practical on a
> 64bit machine (which notably does not include EC2 "small" instances) or
> relatively small datasets. "auto", the safe choice, will enable mmapping on a
> 64bit JVM. Other values are "mmap", "mmap_index_only" (which may allow you to
> get part of the benefits of mmap on a 32bit machine by mmapping only index
> files) and "standard". (The buffer size settings that follow only apply to
> standard, non-mmapped i/o.)
> "
> The actual code referring to "mmap_index_only" is in
> src/java/org/apache/cassandra/config/DatabaseDescriptor.java :
> "
> public static enum DiskAccessMode {
> auto,
> mmap,
> mmap_index_only,
> standard,
> }
> ...
> private static DiskAccessMode diskAccessMode;
> private static DiskAccessMode indexAccessMode;
> ...
> if (diskAccessMode == DiskAccessMode.auto)
> {
> diskAccessMode = System.getProperty("os.arch").contains("64")
> ? DiskAccessMode.mmap : DiskAccessMode.standard;
> indexAccessMode = diskAccessMode;
> logger.info("Auto DiskAccessMode determined to be " +
> diskAccessMode);
> }
> else if (diskAccessMode == DiskAccessMode.mmap_index_only)
> {
> diskAccessMode = DiskAccessMode.standard;
> indexAccessMode = DiskAccessMode.mmap;
> }
> else
> {
> indexAccessMode = diskAccessMode;
> }
> "
> As indicated by this snippet, IndexAccessMode is set to "mmap" if
> "mmap_index_only" is set in the conf file. However it does not appear that
> IndexAccessMode or getIndexAccessMode() are used by any other cassandra code.
> "
> ~/repos/cassandra$ grep -ri indexAccessMode .
> ./src/java/org/apache/cassandra/config/DatabaseDescriptor.java: private
> static Config.DiskAccessMode indexAccessMode;
> ./src/java/org/apache/cassandra/config/DatabaseDescriptor.java:
> indexAccessMode = conf.disk_access_mode;
> ./src/java/org/apache/cassandra/config/DatabaseDescriptor.java:
> indexAccessMode = Config.DiskAccessMode.mmap;
> ./src/java/org/apache/cassandra/config/DatabaseDescriptor.java:
> indexAccessMode = conf.disk_access_mode;
> ./src/java/org/apache/cassandra/config/DatabaseDescriptor.java: public
> static Config.DiskAccessMode getIndexAccessMode()
> ./src/java/org/apache/cassandra/config/DatabaseDescriptor.java: return
> indexAccessMode;
> "
> If I understand the code correctly, this means that setting DiskAccessMode to
> "mmap_index_only" is functionally the same as setting it to "standard." As
> people might be tempted to try/test "mmap_index_only" as a mode in-between
> "standard" and "mmap" in order to mitigate concerns about
> https://issues.apache.org/jira/browse/CASSANDRA-1214, it would probably be
> good to either complete the feature or remove the configuration option. I am
> willing to submit a patch for the latter and fix the docs if that's the
> decision.
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