Hi Eric,
thank you for your reply.

The shared storage will have LVM on top and each node will have RW access only to its own LVs (actually, these will be the virtual disks of the vms running on each node), this is meant to say that each node is likely going to access different data -- unless a vm is migrated between nodes (??). Does this change anything?

Unfortunately, exporting luns via iSCSI/AoE is not an option since I'm going to use a DELL VRTX and the storage is provided by a DAS, where there will be both rotational and solid-state disks. 

Regards,
Michele

Il giorno 26/set/2016, alle ore 00:47, Eric Wheeler [off-list ref] ha scritto:

quoted

On Sun, 25 Sep 2016, Michele wrote:

Hello everybody,

I have a KVM virtualization cluster (3 nodes) which shares the same 
storage. It would be possible to attach a caching device on each node to 
that storage?

Let me explain better: I intend to create a raid-0 with two SSDs on each 
node as WT caching devices, then initialize the shared storage as bcache 
backing device, and then attach the caching device on each node to it.

Is this going to work?

No.  Even in writethrough mode, remote writes of one node would hit the 
lower layer creating stale writethrough caches on the other nodes---that 
is unless you would like to implement distributed cache coherency using 
DLM.  That would be quite interesting!

You could share bcache0 to multiple nodes if you control its export via 
iSCSI or AoE and place a shared filesystem (gfs, ocfs, cLVM, etc) atop of 
the exported volume.  I'm not sure if it has been tested, but it would be 
neat to get feadback on the subject.

--
Eric Wheeler

quoted

Thanks,
Michele


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