[Gluster-devel] Question about random and rr scheduler

[Craig Tierney]

Amar S. Tumballi wrote:
> On Mon, Mar 3, 2008 at 4:27 PM, Craig Tierney <Craig.Tierney at noaa.gov>
> wrote:
> 
>> I setting up Gluster (1.3.7) with two servers.  I first
>> tried configuring the clients as round-robin (rr). When I try and write
>> to the filesystem for the first time, all of the files go
>> to the first brick.  Subsequent writes will alternate between
>> the two bricks.  When I try random, the first file is always
>> created on the first brick.  Subsequent writes always go to
>> the first brick (never the second).
>>
> 
> As the name suggests 'random' scheduler just calls random() and it just does
> % with the number of clients. Hence, not much control over it by user side
> right now.
> 

> 
> 
>> What I want is round-robin, or a working random.  However,
>> for round-robin to work for me, I need the chosen server to
>> be random, not always the first one.
>>
>> In the long-term, it wouldn't really matter because everything
>> would average out.  However, I am creating filesystems that
>> will be temporary, so I need the right behavior in the short term.
>>
>> Should random do what I need?  Should I look in the code
>> and see how to get the Round-Robin schedule to start with
>> a random index?
>>
> 
> Just initialize index variable in rr scheduler to start with a random
> number. should not be much of a work..
> 
> 


I modified the rr scheduler to use a random index at initialization.
I like the behavior much better.


--- glusterfs-1.3.7/scheduler/rr/src/rr.c       2007-10-05 05:57:12.000000000 +0000
+++ ../glusterfs-1.3.7/scheduler/rr/src/rr.c    2008-03-04 14:53:10.191215255 +0000
@@ -49,7 +49,10 @@
      trav_xl = trav_xl->next;
    }
    rr_buf->child_count = index;
-  rr_buf->sched_index = 0;
+
+  srandom(time(NULL)); /* Initialize the seed */
+  rr_buf->sched_index = random()%index; /* Randomize the initial index */
+
    rr_buf->array = calloc (index + 1, sizeof (struct rr_sched_struct));
    trav_xl = xl->children;
    index = 0;

Craig
-- 
Craig Tierney (craig.tierney at noaa.gov)