[DRBD-user] multiple resources on a harddisk

[Tim Jackson]

Hi Lars, on Thu, 3 Jun 2004 10:34:15 +0200 you wrote:

> Santiago Ruano Rincón:
> > the drbd.conf says this "two drbd devices on the same spindle (hda)? 
> > performance will be bad..."
> > But I need to have three resources in the same hd, is it not posible?
> > how bad is the performance?
> everything is possible,
> and we even have the sync-group parameter
>     # Resynchronization of all devices in one group runs parallel.
>     # Groups are serialized in ascending order. You should avoid
>     # that devices which lower devices share one and the same physical
>     # disk sync in parallel. 
> yes, maybe I should have mentioned that in the comment you cited, too.
> but it is in the very same file, so what :)

Just to pick up on this: yes, if you would consider mentioning the
sync-group parameter in the "don't do this" comment about multiple devices
on the same spindle, it would be less confusing to people new to DRBD
(which includes me!). As it is, someone reading through the file first
reads about the sync-group parameter but then, in spite of that, later
reads "don't do this", which puts them off (well, at least it did for me).
How about "btw, don't do this unless you have the resources in different
sync-groups"?

So, following on from that, I have a (probably common) situation: two
nodes, a limited number of physical disk slots and lots of services which
can run on either node. Each node can handle all the load on its own, but
as a simple kind of load sharing and to save one machine sitting idle,
would the below configuration work OK and with no worse performance (as
far as DRBD syncing speed is concerned) than having 1 big DRBD resource?
It uses two DRBD resources on the same spindle, but with different
sync-groups:

Heartbeat haresources:

node1 datadisk::drbd0 Filesystem::/dev/nb0::/ha/web httpd
node2 datadisk::drbd1 Filesystem::/dev/nb1::/ha/mysql mysqld

# assume drbd0 contains all web data
resource drbd0 {
  sync-group=0

  on node1 {
    device=/dev/nb0
    disk=/dev/sda1  
  }

  on node2 {
    device=/dev/nb0
    disk=/dev/sda1
  }

}

# assume drbd1 contains all mysql data
resource drbd1 {
  sync-group=1

  on node1 {
    device=/dev/nb1
    disk=/dev/sda2  
  }

  on node2 {
    device=/dev/nb1
    disk=/dev/sda2
  }

}