[sheepdog-users] Understanding number of copies and offline node behaviour

[Gerald Richter - ECOS]

Hi Valerio,

thanks very much for your explanation. This help very much. I have read about this modes in the man page, but unfortunately there is no description what these modes mean (at least in the 0.6 man page).

I have two questions left:

Can I change this mode later on without the need to reformat the cluster when I have more nodes in the cluster (so I save have to export/import several TByte)?

What happens if I use "collie cluster format --unsafe" or " collie cluster format --quorum" and there more nodes down so some data is missing. Does my VM just gets an I/O error on the missing data and will work again if enough nodes are up again or do I risk to loose data in such a situation because the cluster might not recover? (e.g. if I upgrade to 6 nodes and have copies = 3, then sheepdog still works with two nodes, but some data is missing because 4 nodes are down. I don't care that VM are not working in this situation. I just care that I loose no data when the nodes are up and running again)

Thanks & Regards

Gerald


> -----Ursprüngliche Nachricht-----
> Von: sheepdog-users-bounces at lists.wpkg.org [mailto:sheepdog-users-
> bounces at lists.wpkg.org] Im Auftrag von Valerio Pachera
> Gesendet: Freitag, 6. September 2013 15:51
> An: Lista sheepdog user
> Betreff: Re: [sheepdog-users] Understanding number of copies and offline
> node behaviour
> 
> The short answer is that 1 node is enough (if it has enough space to keep at
> least 1 copy for every vdi).
> You have to format the cluster with the right options tough.
> 
> # collie cluster format
> 
> This means the cluster will make 3 copies for each chnunk/object.
> If it can't do that (2 node alive), it consider it a non optima situation and stops
> the cluster.
> 
> The same with --copies 2 the the number of node is less than n copies (2).
> 
> # collie cluster format --unsafe
> 
> This will make 3 copies for each chunk but it will not stop the cluster if the
> number of node is less than n copies.
> It will not bother you even with a single host alive.
> 
> # collie cluster format --quorum
> 
> Sheepdog will keep 3 copies, but it will not stop the cluster if alive node are 2.
> But it will stop the cluster if a single host remains active.
> In general, it will not stop the cluster if nodes > (n copies / 2 ).
> In our case 2 > 3/2.
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