2.5 readpages doubles cifs vfs large file copy performance vs. 2.4

[Steven French]

I forgot to mention something obvious - eventually the read ahead logic of 
linux itself could kick in (see mm/readpages.c) and for this case that 
behavior is important to understand as well as how the filesystem is 
formatted and the properties of the underlying block device.   There is a 
least one filesystem that seems to override these vm operations and could 
do its own readahead (Linux xfs) but most default to the behavior that the 
memory management subsystem defaults to.   In any case - the client read 
size may have less effect on the disk performance than the common Linux 
readahead algorithm in mm/readahead.c but the client read size makes a 
huge difference in single client cases unconstrained disk cases since it 
more efficiently uses the network bandwidth and minimizes server dead time 
when nothing is going on due to client processing delay.

Steve French
Senior Software Engineer
Linux Technology Center - IBM Austin
phone: 512-838-2294
email: sfrench at-sign us dot ibm dot com