On Tuesday 31 January 2006 9:08 am, Markus Hochholdinger wrote:
> so clvm is a replacement for gnbd but not for network block device?
no, CLVM is an option of LVM:
under LVM, all the volumes metadata (that is, the mapping info) is stored in
the block devices themselves. this way, if you have a host with some disks,
you join them, and split in any way using LVM. later on, you shut down the
machine, take off the disks and install them in another Linux box. on the
new host, you just do "vgscan", and presto! all the LVs are there! not so
surprising... but that means that the joining, splitting, and all is in the
block devices.
now... what happens if the block devices are on a SAN with several hosts
seeing them?
using LVM, you first turn on ONE host; configure ALL the LVs as you wish.
then turn on all hosts, and everyone will do vgscan as part of the startup.
from then on, all hosts will see the same LVs. but if you then create,
delete,extend a new LV, or add, remove, move, etc. a PV... then the other
hosts will have inconsistent mappings until they do vgscan again.
that's why in LVM docs it says that to modify a shared volume group you have
to unmount and "vgchange -an" all from all hosts but one (effectively
disconnecting them from the SAN), do all changes on ONE host, and then do
"vgchange -ay" on the other hosts.
IOW: with shared drives, you lose the ability to do online modifications to
the volume group.
that's where CLVM enters.
the GFS stack includes lots of kernel modules and daemons to manage the
"Cluster" concept, where there are some shared resources and locking.
there's a kind of plugin for LVM that uses those systems to distribute and
synchronize all the LVM managing.
so, if you configure the 'cluster' system, the CLVM managing is really simple:
all the LVM commands work as expected, and do all the communications needed
to keep the device mappings consistent in all hosts.
you just change anything on a shared volume group, and all hosts are notified
and transparently reread the device mappings.
--
Javier
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