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

Lentes, Bernd bernd.lentes at helmholtz-muenchen.de
Thu Nov 24 16:32:12 UTC 2011

Digimer wrote:

> > 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.

Yes, that's logical.

> > 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.

Strange people who have ideas like that.

> > 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.c
> fm?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.

I ordered one.

> > 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.

I don't have a filesystem. I will install the vm's (using KVM) in bare partitions (lv's).
Is that a problem ?
I got recommendations this is faster than installing them in partitions with a filesystem.

> > 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.

But you said "So DLM thinks that Node 1 was fenced, clears it's old locks and gives a
new one to Node 2" How can node 1 get access after unfreezing, when the lock is cleared ?

> 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. :)

The situation that two nodes offer the same service should normally be prevented by the CRM.

Thanks for your very detailed answer.


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