[DRBD-user] Cluster filesystem question

Andreas Hofmeister andi at collax.com
Fri Nov 25 21:11:39 CET 2011

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

On 25.11.2011 17:08, John Lauro wrote:

> What I’m not sure about from the examples… Can you then add more
> GFS/OCFS2 nodes (ie: a 3^rd and 4^th node) that have no local disk as
> part of the SAN cluster, but instead talk drbd to the first two nodes?
> Or would you have to run a shared failover service such as NFS on top of
> the two node cluster if you need multiple hosts accessing it?

For OCFS or GFS, all nodes need access to the same block device at the 
same time. In a two-node setup, you can easily use DRBD in dual-primary 
mode to provide such a shared block device.

For more than two nodes, you need to export the block device provided by 
DRBD by a different method (such as iSCSI), since DRBD (normaly) 
supports only two nodes to share a block device.

Depending on your needs, you can either go for an active-passive or an 
active-active export of the DRBD device(s).

Active-passive is way easier to implement. Try Google for "DRBD iSCSI" 
and you'll find some interesting readings on that. Generally this route 
involves DRBD in single-primary mode, iSCSI target/initiator 
implementations, a method to share an IP addresses and a cluster manager 
to coordinate all those things.

The active-active route seems to be somewhat harder - or at least I did 
not find any useful references. A possible implementation would likely 
include all of the above, except that one would use SCSI-multipath 
instead of sharing a single IP address to access the iSCSI targets.

@list: did anybody try such a thing ?

> Mainly ask because I assume recovery time from failover would be much
> quicker with a cluster filesystem (doesn’t have to fsck/remount), and
> reads (would?) be split over the two hosts instead of just one, so it
> should be slightly faster with two hosts serving read I/O requests
> instead of one.

In an active-passive setup, you won't get any performance gains. Whether 
you can see any throughput improvements with an active-active setup 
would basically depend on your network setup.

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