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


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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