If I understandard correctly the docs for current are based on current stable 0.20.2. Current stable does not use fs.inmemory.size.mb.
http://hadoop.apache.org/common/docs/current/cluster_setup.html. Under real world configurations
conf/core-site.xml fs.inmemory.size.mb 200 Larger amount of memory allocated for the in-memory file-system used to merge map-outputs at the reduces.
As to "io.sort.factor and io.sort.mb"
They both appear in mapred-default.xml
[edward@ec src]$ grep -R "io.sort.factor" */*.xml
They should be in core-default.xml (only), or in both core-default.xml and mapred-default.conf.
Think about the end user. An end user might read a blog that states, "io.sort.factor is a magic tune set this to XXXX for awesome performance". Which file should end user put this variable in?
grep -R "io.sort.factor" */*.xml
End user thinks, "Since I found this variable in mapred-default.xml it makese sense that I should override it in mapred-site.xml"
The user puts the variable in the wrong place, because end user has no (easy) way of knowing that SequenceFile uses io.sort.factor or io.sort.mb. Does that make sense?