Re: [Xen-users] XCP and Openfiler as the storage

[Eduardo Bragatto <eduardo@xxxxxxxxxxxx>]

Hi,

according to FG he does share the LUNs and it is allowed. I had the  
same wrong impression that sharing a LUN would cause data corruption,  
but that's not true for all cases.
According to Wikipedia: http://en.wikipedia.org/wiki/ISCSI

"In enterprise deployments, LUNs usually represent slices of large  
RAID disk arrays, often allocated one per client. iSCSI imposes no  
rules or restrictions on multiple computers sharing individual LUNs;  
it leaves shared access to a single underlying filesystem as a task  
for the operating system."
Pay attention to what it says here: it leaves shared access to a  
single underlying filesystem as a task for the operating system,
Which means if you create an EXT3 filesystem (which can not be shared)  
on a LUN and mount it and use it on multiple servers, data will become  
corrupted very soon.
However, if you create a GFS filesystem on that same LUN, it will run  
fine from multiple systems.
Therefore, it's clear that the LUN can be shared, as long as the layer  
on top of it can handle being shared.
From what I understand, XCP will handle the entire LUN as a shared  
storage, and will create separate LVMs for each VM without any  
problem. I believe you would only have a problem if you decided yo  
actually *mount* the filesystems inside those LVMs in multiple servers  
-- but as long as you don't, and XCP seems to handle that, you won't  
have problems.
My conclusion is a bit speculative as I'm still researching the  
subject and I'm assuming XCP takes care of avoiding any race  
conditions when mounting the filesystems from the shared LUN (i.e. not  
allowing two hypervisors to attempt to mount them at the same time and  
properly unmount / mount during a live migration). If anyone can shed  
any lights here it would be very much appreciated.
According to FG he has been using XCP with two big shared LUNs (both >  
1TB) for over one year with no problems.
Best regards,
Eduardo Bragatto

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