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I have done a little test for the image download and upload. I created an 
API for the image access, containing copyFrom and sendTo. I moved the 
image download and upload code from XenApi into the implementation for 
Http with some modifications, and the code worked for libvirt as well. 
copyFrom means to download the image and return the image data, and 
different hypervisors can choose to save it in a file or import it to the 
datastore; sendTo is used to upload the image and the image data is passed 
in as a parameter.

I also did an investigation about how each hypervisor is doing the image 
upload and download.

For the download:
libvirt, hyper-v and baremetal use the code image_service.download to 
download the image and save it into a file.
vmwareapi uses the code image_service.download to download the image and 
import it into the datastore.
XenAPi uses image_service.download to download the image for VHD image.

For the upload:
They use image_service.upload to upload the image.

I think we can conclude that it is possible to have a common image 
transfer library with different implementations for different protocols.
This is a small demo code for the library: 
https://review.openstack.org/#/c/90601/(Jay, is it close to the library as 
you mentioned?). I just replaced the upload and download part with the 
http implementation for the imageapi and it worked fine.

Best wishes,
Vincent Hou (侯胜博)

Staff Software Engineer, Open Standards and Open Source Team, Emerging 
Technology Institute, IBM China Software Development Lab

Tel: 86-10-82450778 Fax: 86-10-82453660
Notes ID: Sheng Bo Hou/China/IBM at IBMCN    E-mail: sbhou at cn.ibm.com 
Address:3F Ring, Building 28 Zhongguancun Software Park, 8 Dongbeiwang 
West Road, Haidian District, Beijing, P.R.C.100193
地址:北京市海淀区东北旺西路8号中关村软件园28号楼环宇大厦3层 邮编：100193



Solly Ross <sross at redhat.com> 
2014/04/25 01:46
Please respond to
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Subject
Re: [openstack-dev] [Nova][blueprint] Accelerate the booting process of a 
number of vms via VMThunder






Something to be aware of when planing an image transfer library is that 
individual drivers
might have optimized support for image transfer in certain cases 
(especially when dealing
with transferring between different formats, like raw to qcow2, etc). This 
builds on what
Christopher was saying -- there's actually a reason why we have code for 
each driver.  While
having a common image copying library would be nice, I think a better way 
to do it would be to
have some sort of library composed of building blocks, such that each 
driver could make use of
common functionality while still tailoring the operation to the quirks of 
the particular drivers.

Best Regards,
Solly Ross

----- Original Message -----
From: "Christopher Lefelhocz" <christopher.lefelhoc at RACKSPACE.COM>
To: "OpenStack Development Mailing List (not for usage questions)" 
<openstack-dev at lists.openstack.org>
Sent: Thursday, April 24, 2014 11:17:41 AM
Subject: Re: [openstack-dev] [Nova][blueprint] Accelerate the booting 
process of a number of vms via VMThunder

Apologies for coming to this discussion late...

On 4/22/14 6:21 PM, "Jay Pipes" <jaypipes at gmail.com> wrote:

>
>Right, a good solution would allow for some flexibility via multiple
>transfer drivers.

+1. In particular I don't think this discussion should degenerate into
zero-copy vs. pre caching.  I see both as possible solutions depending on
deployer/environment needs.

>
>> Jay Pipes has suggested we figure out a blueprint for a separate
>> library dedicated to the data(byte) transfer, which may be put in oslo
>> and used by any projects in need (Hoping Jay can come in:-)). Huiba,
>> Zhiyan, everyone else, do you think we come up with a blueprint about
>> the data transfer in oslo can work?
>
>Yes, so I believe the most appropriate solution is to create a library
>-- in oslo or a standalone library like taskflow -- that would offer a
>simple byte streaming library that could be used by nova.image to expose
>a neat and clean task-based API.
>
>Right now, there is a bunch of random image transfer code spread
>throughout nova.image and in each of the virt drivers there seems to be
>different re-implementations of similar functionality. I propose we
>clean all that up and have nova.image expose an API so that a virt
>driver could do something like this:
>
>from nova.image import api as image_api
>
>...
>
>task = image_api.copy(from_path_or_uri, to_path_or_uri)
># do some other work
>copy_task_result = task.wait()
>
>Within nova.image.api.copy(), we would use the aforementioned transfer
>library to move the image bits from the source to the destination using
>the most appropriate method.

If I understand correctly, we'll create some common library around this.
It would be good to understand the details a bit better.  I've thought a
bit about this issue.  The one area that I get stuck at is providing a
common set of downloads which work across drivers effectively.  Part of
the reason there are a bunch or random image transfers is historical, but
also because performance was already a problem.  Examples include:
transferring to compute first then copying to dom0 causing performance
issues, needs in some drivers to download image completely to validate
prior to putting in place, etc.

It may be easy to say we'll push most of this to the dom0, but I know for
Xen our python stack is somewhat limited so that may be an issue.

By the way we've been working on proposing a simpler image pre caching
system/strategy.  It focuses specifically on the image caching portion of
this discussion.  For those interested, see the nova-spec
https://review.openstack.org/#/c/85792.  We'd like to leverage whatever
optimized image download strategy is available.

Christopher 


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