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From "Matthew Hartmann" <>
Subject XenServer & VM Snapshots
Date Mon, 03 Dec 2012 17:08:08 GMT
Hello! I'm hoping someone can help me troubleshoot the following issue:


I have a client who has a 960G data volume which contains their VM's
Exchange Data Store. When starting a snapshot, I found that a process is
started on one of my Compute Nodes titled "sparse_dd". I found that this
process is then sending the output of "sparse_dd" through another Compute
Node's xapi before placing it into the "snapshot store" on Secondary
Storage. It appears that this is part of the bottle neck as all of our
systems are connected via gigabit link and should not take 15+ hours to
create a snapshot. The following is the behavior that I have analyzed from
within my environment:


1)     Snapshot is started (either via Manual or Scheduled).

2)     Compute Node 1 "processes the snapshot" by exposing the VDI which
"sparse_dd" then creates a "thin provisioned" snapshot.

3)     The output of sparse_dd is delivered over HTTP to xapi on Compute
Node 2 where the Management Server mounted Secondary Storage.

4)     Compute Node 2 (receiving the snapshot via xapi) stores the snapshot
in the Secondary Storage mount point.


Based on the behavior, I have devise the following logic that I believe
CloudStack is utilizing:


1)     CloudStack creates a "snapshot VDI" via XenServer Pool Master's API.

2)     CloudStack finds a Compute Node that can mount Secondary Storage.

3)     CloudStack finds a Compute Node that can run "sparse_dd".

4)     CloudStack uses available Compute node to output the VDI to xapi on
the Compute Node that mounted Secondary Storage.


I must mention that the same Compute Node that ran sparse_dd or mounted
Secondary Storage is not always the same. It appears the Management Server
is simply round-robining through the list of Compute Nodes and using the
first one that is available.


Does anyone have any input on the issue I'm having or analysis of how
CloudStack/XenServer snapshots operate?










Matthew Hartmann
Systems Administrator | V: 812.378.4100 x 850 | E:

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