[Linux-cluster] Behavior of "statfs_fast" settune

[Mathieu Avila]

Le Wed, 16 Jan 2008 11:25:38 -0500,
Wendy Cheng <wcheng at redhat.com> a écrit :

> Wendy Cheng wrote:
> > Mathieu Avila wrote:
> >> Hello GFS developers,
> >>
> >> I am in the process of evaluating the performance gain of
> >> the "statfs_fast" patch.
> >> Once the FS is mounted, I perform "gfs_tool settune ...." and then
> >> i measure the time to perform "df" on a partially filled FS. The
> >> time is almost the same, "df" returns almost instantly, with a
> >> value really near the truth, and progressively reaching the true
> >> one.
> >>
> >> But I have noticed that when the FS size increases, the time to
> >> perform "gfs_tool settune ...." increases dramatically. In fact,
> >> after a few measures, it appears that the time to perform "df"
> >> without fuzzy statfs is the same as the time to activate fuzzy
> >> statfs. 
> > In theory, this shouldn't happen. Are you on RHEL 4 or RHEL 5 ? And 
> > what is the FS size that causes this problem ?
> >
> I just did a quick try. It doesn't happen to me. By reading your
> note, were you *repeatedly* issuing "gfs_tool settune .." then
> followed by "df" ? Remember the "settune" is expected to be run
> *once* right after the particular GFS filesystem is mounted. You
> certainly *can* run it multiple times. It won't hurt anything.
> However, each time the "settune" is invoked, the code has to perform
> a regular "df" (i.e. that's the way it initializes itself). I suspect
> this is the cause of your issue. Let me know either way.
> 

I am using "cluster-1.03" with the statfs_fast patch from:
http://www.redhat.com/archives/cluster-devel/2007-March/msg00124.html
(has this been changed after ?)
All this on a Centos 5.

My use case is :
 * mkfs of a volume
 * mount on all 6 nodes 
 * timing of "settune statfs_fast 1", on all 6 nodes. 
 * timing of "df" on one node.
All commands are executed immediately one after the other.

So i issued only one "settune", on all nodes, and was expecting it to
return immediately. From what you've just said (settune performing a
real "df"), i guess this behaviour is normal.

I don't understand why it's necessary to perform a real "df" in
"settune". Isn't the licence inode used to store the previous
values of "df" so that it can give an immediate answer to "df", and then
perform a real regular "df" in background to upgrade the "cached df" to
the real value ?

--
Mathieu


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