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XenCenter 7 snapshot file size

Mitch Vega


Simple question....I am trying to determine the file size of a snapshot taken of a XenCenter 7 VM Server VM. Is it possible to do this from XenCenter...or from the CLI? 


I have done some googling and am having a difficult time trying to find an answer to this one. I have not been able to determine where on the host system the snapshots are stored or how to find the file size.  Thanks for any input. 

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The initial snapshot will have the same size as the original storage; subsequent snapshots are incremental changes to the original. And, no, the snapshot size cannot be regulated. Over time, a lot will depend on how much activity there is on storage. Note that the maximum length of a snapshot chair is around 30 snapshots.



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When a VM is initially created it is associated with a single VDI (on all but GFS2 this is a VHD file or a VHD formatted logical volume).


After taking a snapshot the current VDI for the VM is truncated to the smallest size required to contain all the "dirty" blocks that have been written (this may be bigger, sometimes significantly bigger, than the "used" space in the VM) and a new delta VDI is created for new writes to be written to. Technically the snapshot size is the size of the read-only piece as that is what you get if you, for instance, clone from snapshot. The size of the delta VDI depends on the SR type. For block LVM SRs (local LVM, LVM over iSCSI, LVM over HBA, LVM over FCoE) delta VDI will be the full size of the disk allocated to the VM to permit the VM to write to any block in the disk without needing to dynamically resize the logical volume. For file based SRs (Local ext, NFS, SMB3, GFS2) the size of the delta VDI will be just what is required to contain the data that the VM has written since it was created and it will grow on demand as required potentially up to (but not more than) the full size of the disk allocated to the VM.


With a single snapshot the worst case is that 2x full disk size storage is consumed for the VM and for LVM there is never less than 100% full disk size consumed. Scale this dependent on the number of snapshots for the VM.





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