[Linux-cluster] Behavior of "statfs_fast" settune

[Wendy Cheng]

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.

-- Wendy