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Re: [linux-lvm] new to cLVM - some principal questions

On 11/23/2011 10:35 AM, Lentes, Bernd wrote:

Digimer wrote:

On 11/22/2011 02:22 PM, Lentes, Bernd wrote:

i have a bit experience in LVM, but not in cLVM. So i have
some principal questions:
My idea is to establish a HA-Cluster with two nodes. The
ressources which are managed by the cluster are virtual
machines (KVM).
I have a FC SAN, where the vm's will reside. I want to
create vdisks in my SAN which are integrated as a PV in both
hosts. On top of the PV's i will create a VG, and finally
LV's. For each VM one LV.

How are things going with cLVM ? Do i have to create PV ==>
  VG ==>   LV seperately ? Or does cLVM replicate the
information from one host to the other ? So that i have to
create PV, VG and LV only once on the first node and this
configuration is replicated to the second host.

What is about e.g. resizing a LV ? Is this replicated, or
do i have to resize twice, on each host ?

E.g. one host is running VM3 in the corresponding lv3 on
the first host. Is the second host able to access lv3
simultaneously or is there a kind of locking ?

Is it possible to run some vm's on the first host and
others on the second (as a kind of load-balancing) ?

Is it possible to perform a live-migration from one host to
the other in this scenario ?

I will not install a filesystem in the lv's, because i got
recommendations to run the vm's in bare partitions, this
would be faster.

Thanks for any eye-opening answer.


Clustered LVM is, effectively, just normal LVM with external
locking using DLM. Once built, anything you do on one node
will be seen
immediately on all other nodes.

Mount your iSCSI target as your normally would on all nodes. On one
node, with clvmd running, 'pvcreate /dev/foo' then 'vgcreate
-c y -n bar
/dev/foo'. If you then run 'vgscan' on all other nodes,
you'll see the
VG you just created.

Be absolutely sure you configure fencing in your cluster! If a node
falls silent, it must be forcibly removed from the cluster before any
recovery can commence. Failed fencing will hang the cluster, and
short-circuited fencing will lead to corruption.

Finally, yes, you can do live migration between nodes in the same
cluster (specifically, they need to be in the same DLM lockspace).

I use clvmd quite a bit, feel free to ask if you have any more
questions. I also have an in-progress tutorial using clvmd on
DRBD, but
you could just replace "/dev/drbdX" with the appropriate iSCSI target
and the rest is the same.


Hi Digimer,

we met already on the DRBD-ML.
clvmd must be running on all nodes ?

Yes, but more to the point, they must also be in the same cluster. Even more specifically, they must be in the same DLM lockspace. :)

I'm planning to implement fencing. I use two HP Server which support iLO.

Good, fencing is required. It's a good idea to also use a switched PDU as a backup fence device. If the iLO loses power (ie, blown power supply or failed BMC), the fence will fail. Having the PDU provides an alternative method to confirm node death and will avoid blocking. That is, when a fence is pending (and it will wait forever for success), DLM will not give out locks so your storage will block.

Using this i can restart a server when the OS is not longer accessible.

The cluster, fenced specifically, will do this for you.

I think that's a kind of STONITH. Is that what you describe with "short-circuited fencing" ?

Fencing and Stonith are two names for the same thing; Fencing was traditionally used in Red Hat clusters and STONITH in heartbeat/pacemaker clusters. It's arguable which is preferable, but I personally prefer fencing as it more directly describes the goal of "fencing off" (isolating) a failed node from the rest of the cluster.

To "short circuit" the fence, I mean return a success message to fenced without actually properly fencing the device. This is an incredibly bad idea that I've seen people try to do in the past.

You recommend not using a STONITH method ? What else can i use for fencing ?

I generally use a mix of IPMI (or iLO/RSA/DRAC, effectively the same thing, but vendor-specific) as my primary fence device because it can confirm that the node is off. However, as mentioned above, it will fail if the node it is in dies badly enough.

In that case, a switched PDU, like the APC 7900 (http://www.apc.com/products/resource/include/techspec_index.cfm?base_sku=AP7900) makes a perfect backup. I don't use it as primary though because it can only confirm that power has been cut to the specified port(s), not that the node itself is off, leaving room for configuration or cabling errors returning false-positives. It is critical to test PDU fence devices prior to deployment and to ensure that cables are then never moved around after.

What is about concurrent access from both nodes to the same lv ? Is that possible with cLVM ?

Yes, that is the whole point. For example, with a cluster-enabled VG, you can create a new LV on one node, and then immediately see that new LV on all other nodes.

Keep in mind, this does *not* magically provide cluster awareness to filesystems. For example, you can not use ext3 on a clustered VG->LV on two nodes at once. You will still need a cluster-aware filesystem like GFS2.

Does cLVM sync access from the two nodes, or does it lock the lv so that only one has exclusive access to the lv ?

When a node wants access to a clustered LV, it requests a lock from DLM. There are a few types of locks, but let's look at exclusive, which is needed to write to the LV (simplified example).

So Node 1 decides it wants to write to an LV. It sends a request to DLM for an exclusive lock on the LV. DLM sees that no other node has a lock, so the lock is granted to Node 1 for that LV's lockspace. Node 1 then proceeds to use the LV as if it was a simple local LV.

Meanwhile, Node 2 also wants access to that LV and asks DLM for a lock. This time DLM sees that Node 1 has an exclusive lock in that LV's lockspace and denies the request. Node 2 can not use the LV.

At some point, Node 1 finishes and releases the lock. Now Node 2 can re-request the lock, and it will be granted.

Now let's talk about how fencing fits;

Let's assume that Node 1 hangs or dies while it still holds the lock. The fenced daemon will be triggered and it will notify DLM that there is a problem, and DLM will block all further requests. Next, fenced tries to fence the node using one of it's configured fence methods. It will try the first, then the second, then the first again, looping forever until one of the fence calls succeeds.

Once a fence call succeeds, fenced notifies DLM that the node is gone and then DLM will clean up any locks formerly held by Node 1. After this, Node 2 can get a lock, despite Node 1 never itself releasing it.

Now, let's imagine that a fence agent returned success but the node wasn't actually fenced. Let's also assume that Node 1 was hung, not dead.

So DLM thinks that Node 1 was fenced, clears it's old locks and gives a new one to Node 2. Node 2 goes about recovering the filesystem and the proceeds to write new data. At some point later, Node 1 unfreezes, thinks it still has an exclusive lock on the LV and finishes writing to the disk.

Voila, you just corrupted your storage.

You can apply this to anything using DLM lockspaces, by the way.

Thanks for your answer.

Happy to help. :)

E-Mail:              digimer alteeve com
Freenode handle:     digimer
Papers and Projects: http://alteeve.com
Node Assassin:       http://nodeassassin.org
"omg my singularity battery is dead again.
stupid hawking radiation." - epitron

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