Use of READDIRPLUS on large directories

[NeilBrown <neilb@xxxxxxx>]

Hi Trond / Bryan et al.

Now that openSUSE 11.4 is out I have started getting a few reports
of regressions that can be traced to 

commit 0715dc632a271fc0fedf3ef4779fe28ac1e53ef4
Author: Bryan Schumaker <bjschuma@xxxxxxxxxx>
Date:   Fri Sep 24 18:50:01 2010 -0400

    NFS: remove readdir plus limit
    
    We will now use readdir plus even on directories that are very large.
    
    Signed-off-by: Bryan Schumaker <bjschuma@xxxxxxxxxx>
    Signed-off-by: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>


This particularly affects users with their home directory over
NFS, and with largish maildir mail folders.

Where it used to take a smallish number of seconds for (e.g.)
xbiff to start up and read through the various directories, it now
takes multiple minutes.

I can confirm that the slow down is due to readdirplus by mounting the
filesystem with nordirplus.


While I can understand that there are sometime benefits in using
readdirplus for very large directories, there are also obviously real
costs.  So I think we have to see this patch as a regression that should
be reverted.


It would quite possibly make sense to create a tunable (mount option or
sysctl I guess) to set the max size for directories to use readdirplus,
but I think it really should be an opt-in situation.

[[ It would also be really nice if the change-log for such a significant
change contained a little more justification.... :-(   ]]

Thoughts?

Thanks,
NeilBrown
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