[Gluster-users] Block replication with glusterfs for NFS failover

[Brian Candler]

On Wed, Oct 24, 2012 at 11:19:13AM +0200, Runar Ingebrigtsen wrote:
> >GlusterFS replication works at a different layer: each glusterfs brick sits
> >on top of a local filesystem, and the operations are at the level of files
> >(roughly "open file named X", "seek", "read file", "write file") rather than
> >block-level operations.
> 
> Hm, does this mean the whole file will be replicated each time it
> changes?

Nope. Gluster works at the POSIX filesystem layer, so commands like
"seek(x); write(data)" would replicate as the same commands to both bricks.

There used to be an issue with healing, i.e. fixing up replicas after they
have been offline for a while.  Prior to gluster 3.3 this involved locking
the whole file, which if it was a VM image would make it unavailable until
healing was complete.  Gluster 3.3.x does healing across ranges instead.

However gluster 3.3.x is still not ideal as a VM backing store, because of
the performance issues of going via the kernel and back out through the FUSE
layer.  There are bleeding-edge patches to KVM which allow it to use
libglusterfs to talk directly to the storage bricks, staying in userland:
http://lists.gnu.org/archive/html/qemu-devel/2012-06/msg01745.html

Or you could try using NFS to gluster's NFS server. Or you can boot from a
gluster image, but mount a gluster volume within the VM for application
data.

Regards,

Brian.