[DRBD-user] Food for thoughts please - DRBD GFS2 CLVM etc

Theophanis Kontogiannis theophanis_kontogiannis at yahoo.gr
Wed Mar 12 16:16:51 CET 2008

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

Hello All,

I am sending this e-mail to the list to ask for food for thoughts.

I have two same servers based on AMD x2 64bit loaded with CENTOS5. 
Memory and resources are not a problem since I begun with small systems and
can expand up to 8 SATA disks, 2 IDE, 16GB RAM.

Now I have the same setup for HDs on both of them. One 80Gbyte and one
320Gbyte both IDE and sharing the /boot and / in RAID-1. 
Also the left over space on both of them will be used as DRBD devices (that
is /dev/hda4 --> /dev/drbd0 and /dev/hdb4 --> /dev/drbd1) in primary/primary

What I plan to have as active applications are:

2.	MySQL Production
3.	MySQL Development
4.	Apache
5.	storage for documents 
6.	and in the future compilation and execution of MPI based code
I have thought about the possible implementation and I end up with two


/dev/drbd0 and /dev/drbd1 are members of one physical volume. Then running
cman, gfs2 and clvmd, I create on large logical volume out of them, and then
I format it as GFS2. All the disk space is simultaneously available and
mounted as GFS on both servers. I create the directories with proper
permissions that the applications will use. Using system-config-cluster or
COGNA, I configure the services I want, and "run" them on the nodes I

It looks that this scenario gives me the good option of live backup on
either node (since the whole FS is mounted at the same time to both nodes),
simple application migration if a node fails and easy expansion of the file
system (I add on the PV the new /dev/drbd when I add a new disk and then
expand gfs2). That way I keep a coherent image of my files and at the same
time I can keep on adding disks, applications or whatever easily. 

Now it seems to me that the problem with this setup, is how to make
exploitation of the cluster manager (RHEL 5), in a way that will auto
migrate the applications if the DRBD fails on one node. Is there any way to
exploit the CMAN instead of heartbeat to manage first of all the underneath
DRBD devices? And then how will cman be aware that it must not start the
application because the FS could not be mounted? Maybe use a custom script
as resource?


Again /dev/drbdX are members of the same physical volume. 

However this time, I create separate LVs formatted with ext3, one for every
application that will run. Then with CMAN I create as resource also the LV
to be mounted prior to the start of the service that is using it. In this it
looks like the question of the previous scenario, on how I will control via
CMAN the DRBD devices is some how solved. If it can not mount the LV this
means that there is a problem with DRBD. But in that case the service is
related to prior mounting the respective LV so it will not start.

However I think that this does not scale well and also it looks like I will
have an issue with backup.

For backup I will use an external USB disk connect to one of the server, via
which the already mounted FSs will be backed up!

I appreciate your comments and sharing your experiences.


Theophanis Kontogiannis

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