On 01/14/2011 03:58 PM, Jonathan Tripathy wrote:
On 14/01/11 19:00, Stan Hoeppner wrote:
Jonathan Tripathy put forth on 1/13/2011 4:17 PM:
Regarding the servers, I was thinking of having a 2 node drbd cluster (in active+standby), which would export a single iSCSI LUN. Then, I would have a 2 node dovecot+postfix cluster (in active-active), where each node would mount the same LUN (With GFS2 on top). This is 4 servers in total (Well, 4 VMs running on 4 physically separate servers). Something you need to consider very carefully:
drbd is a kernel block storage driver. You run in ON a PHYSICAL cluster node, and never inside a virtual machine guest. drbd is RAID 1 over a network instead of a SCSI cable. Is is meant to protect against storage and node failures. This is how you need to look at drbd. Again, DO NOT run DRBD inside of a VM guest. If you have a decent background in hardware and operating systems, it won't take you 30 seconds to understand what I'm saying here. If it takes you longer, then consider this case:
You have a consolidated Xen cluster of two 24 core AMD Magny Cours servers each with 128GB RAM, an LSI MegaRAID SAS controller with dual SFF8087 ports backed by 32 SAS drives in external jbod enclosures setup as a single hardware RAID 10. You spread your entire load of 97 virtual machine guests across this two node farm. Within this set of 97 guests, 12 of them are clustered network applications, and two of these 12 are your Dovecot/Postfix guests.
If you use drbd in the way you currently have in your head, you are mirroring virtual disk partitions with drbd _SIX times_ instead of once. Here, where you'd want to run drbd is within the Xen hypervisor kernel. drbd works at the BLOCK DEVICE level, not the application layer.
Eric already mentioned this once. Apparently you weren't paying attention.
I'm sorry I don't follow this. It would be appreciated if you could include a simpler example. The way I see it, a VM disk is just a small chunck "LVM LV in my case" of a real disk.
Perhaps if you were to compare and contrast a virtual disk to a raw disk, that would help. If you wanted to use drbd with a raw disk being accessed via a VM guest, that would probably be all right. Might not be "supported" though.
-- -Eric 'shubes'