[DRBD-user] multiple drbd devices on a raid array

[Todd Denniston]

Lars Ellenberg wrote:
> 
> / 2004-07-06 10:21:44 -0700
> \ Sean Dwyer:
> > I have a seven disk raid array that is split into six partitions, and
> > I want to mirror each partition with drbd. I see that drbd.conf
> > discourages multiple drbd devices on one spindle, but how about on one
> > raid array?
> > Is performance likely to be an issue?
> 
> test it :)
> 
> > I am using drbd 0.6.12.
> 
> there is the "sync-group" parameter, too,
> to serialize the synchronization of devices.
Sean,
I have a Raid 5 array with 5 disks and several partitions, and Lars answered
my query the same.
When I followed Lars advice I found that using the sync-group mitigated the
problems.
I have tried setting 2 of my logical devices in the same sync group, and found
this is a bad idea, it looked like the net through put was overall a bit
higher and more constant, but it took longer for the sync to occur.
My experience is put each partition of the a single physical raid array in a
separate sync-group and things will go ok.
It would be nice if the comment in the example 0.6.12 drbd.conf example
indicated something better like:

 #echo "*****************" ; diff -u drbd.conf.orig drbd.conf ; echo
"***********"
*****************
--- drbd.conf.orig      2004-07-06 13:33:13.000000000 -0500
+++ drbd.conf   2004-07-06 13:40:32.000000000 -0500
@@ -156,6 +156,7 @@
     ko-count    = 4     # if some block send times out this many times,
                        # the peer is considered dead, even if it still
                        # answeres ping requests
+    sync-group = 0      #sync this disk first to reduce thrashing while
syncing.
   }
 
   on tcube1 {
@@ -179,6 +180,10 @@
   protocol=C
   fsckcmd=fsck -p -y
 
+  net {
+    sync-group = 1
+  }
+
   on tcube1 {
     device=/dev/nb1
 
@@ -187,6 +192,9 @@
         # on the same spindle (hda)?  performance will be bad.  if you
         # use several drbd devices, put them on different spindles;
         # different channels/controllers won't be a bad idea for IDE.
+       # putting more than one drbd device on the same spindle kills
+       # resyncronization performance. this can be mitigated somewhat by 
+       # putting each device in a different sync-group. 
 
     disk=/dev/hda7
     address=10.1.1.31
***********

-- 
Todd Denniston
Crane Division, Naval Surface Warfare Center (NSWC Crane) 
Harnessing the Power of Technology for the Warfighter