config file option DiskAccessMode has no-op option "mmap_index_only"
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Key: CASSANDRA-1241
URL: https://issues.apache.org/jira/browse/CASSANDRA-1241
Project: Cassandra
Issue Type: Bug
Components: Core, Documentation & website
Affects Versions: 0.6.1, 0.6, 0.5, 0.4, 0.3, 0.6.2, 0.6.3, 0.6.4, 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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