Oh snap, that sounds great I didn't know about that.. it makes all
easier. In this scenario only frontend can work with LVM, so no issues
of concurrent change. Only one last think to make it really safe against
that. Is there any way to suppress LVM changes from hosts, make it read
only? And let it RW at frontend?
Thanks
Dne 21.1.2013 18:50, Mihály Héder napsal(a):
Hi,
no, you don't have to do any of that. Also, nebula doesn't have to
care about LVM metadata at all and therefore there is no corresponding
function in it. At /etc/lvm there is no metadata, only configuration
files.
Lvm metadata simply sits somewhere at the beginning of your
iscsi-shared disk, like a partition table. So it is on the storage
that is accessed by all your hosts, and no distribution is necessary.
Nebula frontend simply issues lvcreate, lvchange, etc, on this shared
disk and those commands will manipulate the metadata.
It is really LVM's internal business, many layers below opennebula.
All you have to make sure that you don't run these commands
concurrently from multiple hosts on the same iscsi-attached disk,
because then they could interfere with each other. This setting is
what you have to indicate in /etc/lvm on the server hosts.
Cheers
Mihály
On 21 January 2013 18:37, Miloš Kozák <milos.ko...@lejmr.com> wrote:
Thank you. does it mean, that I can distribute metadata files located in
/etc/lvm on frontend onto other hosts and these hosts will see my logical
volumes? Is there any code in nebula which would provide it? Or I need to
update DS scripts to update/distribute LVM metadata among servers?
Thanks, Milos
Dne 21.1.2013 18:29, Mihály Héder napsal(a):
Hi,
lvm metadata[1] is simply stored on the disk. In the setup we are
discussing this happens to be a shared virtual disk on the storage,
so any other hosts that are attaching the same virtual disk should see
the changes as they happen, provided that they re-read the disk. This
re-reading step is what you can trigger with lvscan, but nowadays that
seems to be unnecessary. For us it works with Centos 6.3 so I guess Sc
Linux should be fine as well.
Cheers
Mihály
[1]
http://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/lvm_metadata.html
On 21 January 2013 12:53, Miloš Kozák <milos.ko...@lejmr.com> wrote:
Hi,
thank you for great answer. As I wrote my objective is to avoid as much
of
clustering sw (pacemaker,..) as possible, so clvm is one of these things
I
feel bad about them in my configuration.. Therefore I would rather let
nebula manage LVM metadata in the first place as I you wrote. Only one
last
thing I dont understand is a way nebula distributes LVM metadata?
Is kernel in Scientific Linux 6.3 new enought to LVM issue you mentioned?
Thanks Milos
Dne 21.1.2013 12:34, Mihály Héder napsal(a):
Hi!
Last time we could test an Equalogic it did not have option for
create/configure Virtual Disks inside in it by an API, so I think the
iSCSI driver is not an alternative, as it would require a
configuration step per virtual machine on the storage.
However, you can use your storage just fine in a shared LVM scenario.
You need to consider two different things:
-the LVM metadata, and the actual VM data on the partitions. It is
true, that the concurrent modification of the metadata should be
avoided as in theory it can damage the whole virtual group. You could
use clvm which avoids that by clustered locking, and then every
participating machine can safely create/modify/delete LV-s. However,
in a nebula setup this is not necessary in every case: you can make
the LVM metadata read only on your host servers, and let only the
frontend modify it. Then it can use local locking that does not
require clvm.
-of course the host servers can write the data inside the partitions
regardless that the metadata is read-only for them. It should work
just fine as long as you don't start two VMs for one partition.
We are running this setup with a dual controller Dell MD3600 storage
without issues so far. Before that, we used to do the same with XEN
machines for years on an older EMC (that was before nebula). Now with
nebula we have been using a home-grown module for doing that, which I
can send you any time - we plan to submit that as a feature
enhancement anyway. Also, there seems to be a similar shared LVM
module in the nebula upstream which we could not get to work yet, but
did not try much.
The plus side of this setup is that you can make live migration work
nicely. There are two points to consider however: once you set the LVM
metadata read-only you wont be able to modify the local LVMs in your
servers, if there are any. Also, in older kernels, when you modified
the LVM on one machine the others did not get notified about the
changes, so you had to issue an lvs command. However in new kernels
this issue seems to be solved, the LVs get instantly updated. I don't
know when and what exactly changed though.
Cheers
Mihály Héder
MTA SZTAKI ITAK
On 18 January 2013 08:57, Miloš Kozák <milos.ko...@lejmr.com> wrote:
Hi, I am setting up a small installation of opennebula with
sharedstorage
using iSCSI. THe storage is Equilogic EMC with two controllers.
Nowadays
we
have only two host servers so we use backed direct connection between
storage and each server, see attachment. For this purpose we set up
dm-multipath. Cause in the future we want to add other servers and some
other technology will be necessary in the network segment. Thesedays we
try
to make it as same as possible with future topology from protocols
point
of
view.
My question is related to the way how to define datastore, which driver
and
TM is the best and which?
My primal objective is to avoid GFS2 or any other cluster filesystem I
would
prefer to keep datastore as block devices. Only option I see is to use
LVM
but I worry about concurent writes isn't it a problem? I was googling a
bit
and I found I would need to set up clvm - is it really necessary?
Or is better to use iSCSI driver, drop the dm-multipath and hope?
Thanks, Milos
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