Automounter kills the inode cache on SunOS!

[Ken Mandelberg]

I've been watching our inode cache with pstat, and noticed that no
matter how large we make it, it fills and then frequently collapses.

The filling is no big surprise. If nothing else the arrival of several
thousand netnews articles a day would do it.  Of course someone doing a
du on the src tree has an even more potent effect.

The periodic collapsing was the surprise to me. I figured the inodes
were stuck in the cache until LRU pushed them out one at a time, or
maybe the file was removed. Neither of these would explain the
collapse.

A little further investigation reveals that the vfs code in SunOS 4.0.X
calls vfs_purge on every mount and umount. This purges the entire ufs
inode cache (and name lookup cache) for all filesystems, not just the
one being mounted/unmounted. In the the case of mount this is necessary
to invalidate any vnodes cached below the mount point. In the case of
umount it would seem that a more careful purge could be done, since its
only the ones in the unmounted filesystem that need to be purged. The
purge is done vnode by vnode at any rate. I may be missing some subtlty
on the umount issue.

Since the automounter is busy mounting/umounting all the time, the
cache is frequently purged. It just takes one user accessing a rarely
used filesystem to kill the cache for the more important ones.

Seems to me like automounter could be a performance problem.
-- 
Ken Mandelberg      | km at mathcs.emory.edu          PREFERRED
Emory University    | {decvax,gatech}!emory!km     UUCP 
Dept of Math and CS | km at emory.bitnet              NON-DOMAIN BITNET  
Atlanta, GA 30322   | Phone: (404) 727-7963