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From Michael Ryan <mpr...@alumni.cmu.edu>
Subject Re: suspending machines in Tashi
Date Sun, 25 Mar 2012 20:26:02 GMT
A couple of things:

1. The current implementation of suspend doesn't use the qcow extension 
for snapshots, it does a "migration" to a file.  In this case, write 
performance would be important if you didn't set stopFirst to True in 
the call to __stopVm.

2. I seem to recall the other issue being the possibility of a seek + 
modify being done to the snapshot.  In the case of Qemu though, we're 
streaming the snapshot out, so this isn't an issue.  I think there was 
also some concern over whether HDFS would support even the streaming 
properly, but since an implementation of the DFS interface using HDFS 
never materialized, I don't think this is a problem (whether or not this 
would still be a problem with HDFS, I don't know).

So I think your solution is a good one, Mike S.

More generally though, if you did need to either preserve immediate 
write speed, allow seek + modify, or something similar, you could 
imagine extending the DFS interface to include a call that provides a 
temporary file with those semantics and then a second call to finalize 
it.  In the VFS case, the two calls could give the path directly and be 
a noop.  In the case of HDFS, you could provide a path to local scratch 
space and then perform the copy in the second call.  This doesn't seem 
to be necessary at this point, but I think that was the original idea 
behind some of the structure.

- Michael

On 3/23/2012 4:41 PM, Kozuch, Michael A wrote:
> RETRACTION:  I thought the copying under discussion was in the migration path.  Somehow
I read "migrate" rather than "suspend".  MichaelS's solution seems reasonable.
>
> Mike
>
> -----Original Message-----
> From: Kozuch, Michael A [mailto:michael.a.kozuch@intel.com]
> Sent: Friday, March 23, 2012 3:48 PM
> To: tashi-dev@incubator.apache.org
> Subject: RE: suspending machines in Tashi
>
> I recall that there was an issue, but I don't recall what it was.
>
> I do agree, though, that two copies seems unnecessary.  Another solution might be to
store the snapshot locally, and then send directly to the local disk on the target client.
 It's possible that this approach would be more scalable in that, if I were to migrate 100
VMs, they wouldn't necessarily all use the same resource.
>
> Mike
>

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