[dm-crypt] LUKS in failover cluster

Sohl, Jacob (LNG-SEA) jacob.sohl at applieddiscovery.com
Wed Oct 12 02:06:16 CEST 2011

> -----Original Message-----
> From: dm-crypt-bounces at saout.de [mailto:dm-crypt-bounces at saout.de] On
> Behalf Of Milan Broz
> Sent: Saturday, October 08, 2011 12:51 AM
> To: dm-crypt at saout.de
> Subject: Re: [dm-crypt] LUKS in failover cluster
> On 10/08/2011 06:52 AM, Arno Wagner wrote:
> >> Also, scalability is a requirement in my design, hence XFS. I was
> >> thinking I needed to use multiple LUKS PVs in LVM to grow the
> >> filesystem. But I would end up with multiple LUKS devices to keep
> track
> >> of. I recently found out that LUKS can resize.
> >
> > LUKS _cannot_ resize. The thing is that LUKS does not care about
> > device size. So if you enlarge a device/partition with an intact
> > LUKS header, "cryptsetup luksOpen" will just map the larger
> > device.
> yes, but crytsetup resize is designed that it it supports open
> (active) LUKS containers. You do not need to activate/deactivate,
> resize operation is enough.
> > If you have multiple devices, you can "slave" the additional ones
> > to a "master" device using something like "decrypt_derived".
> > This just takes the master key from the opened "master" container,
> > runns it though a hash and uses this as key for the "slave"
> > device.
> >
> >> Would it be better to
> >> create one LUKS device on top of LVM? Then create a filesystem on
> that?
> >> (Though that would affect resource dependencies.)
> >
> > Sre you sure you need LVM at all?
> LVM is very useful in such configurations, you can dynamically manage
> LVs over encrypted storage.
> Scripts for managing such stack is not complicated but of course
> it is yet another layer to care of.
> Because you mentioned cluster, I will just repeat
> - if you manage it in HA LVM style (the device
> stack is always active exclusively only on one node) it will work.
> (You just need to create own resource for cluster suite - add LUKS
> to the stack. So cluster suite will handle exclusivity and failover
> itself.)

^ HA LVM is what I will be doing.

> For using LUKS in cluster for shared storage (visible to all nodes)
> - if encryption (mapping) runs in server and plaintext device is
> exported
> and shared to nodes (using iscsi + clvm or so) this will work
> - if you want to run dm-crypt on every node (for the same device,
> IOW export ciphertext device) this is not supported.
> (I think that I said on IRC that with proper used cluster fs and
> flush requests this may work - but it is not tested. The problem is of
> course
> with internal crypto queues so without flushing these queues different
> nodes will see different storage content. And moreover, there is no
> cluster
> infrastructure like clvmd to manage such devices consistently.)

Thank you for clarifying this for me, I didn't really understand in IRC
what you meant about "flush requests". I'm not a programmer and don't
really know low-level hardware stuff that much. It's been a lot of stuff
for me to learn.

> Milan

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