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Re: [libvirt] [Libvirt][Sheepdog]Sheepdog pool/volume storage driver for libvirt problem



On 01/29/2013 10:42 AM, harryxiyou wrote:
> Hi Sebastian, Morita and other developers,

Answering what I can...


> 2,  In the func create/delete/resize volume, we call func
> 'virCommandRun(cmd, NULL);'
> to run these commands, which i think the func send these parameters to daemon
> libvirtd and libvirtd send these parameters to Sheepdog, then Sheepdog
> executes them, right?

Not quite - this is forking and executing an actual command, where the
argv[] of that command was built up by the earlier virCommand* calls on
that cmd object.  There is no intermediate path through libvirtd during
virCommandRun.

It sounds like you are not quite familiar with the libvirt flow of
execution.  When you type a 'virsh' command line, virsh calls an API
from libvirt.so.  For qemu integration, the client side uses the remote
driver, where the code in src/libvirt.c ends up calling the driver in
src/remote/remote_driver.c, and the API ends up bundling the arguments
as an RPC call, which gets sent to the existing libvirtd process.  On
the libvirtd daemon side, the RPC call is received and unbundled, and
fed back into a call into libvirt.so, but this time the driver in
src/qemu/qemu_driver.c handles the callback, and for storage operations
forwards the call onto src/storage/storage_driver.c.  So, the
virCommandRun() you are asking about is already being run in the context
of libvirtd, and is just forking out to an external program.

> 
> 3, In func "virStorageBackendSheepdogCreateVol", it composes cmd like this
> "virCommandPtr cmd = virCommandNewArgList(COLLIE, "vdi", "create",
> vol->name, NULL);"
> then send create parameters. When Sheepdog receive these parameters,
> how Sheepdog
> create this volume? I find Sheepdog create a volume like this.
> $ qemu-img create sheepdog:Alice 256G

This is using 'collie', not 'qemu-img', to create the volume.  That is,
the current sheepdog storage driver is NOT going through qemu, but going
directly to sheepdog.

> 
> 5, Actually, i am not clear about how QEMU receive these parameters
> (like create/delete/snapshot,
> etc) and execute the real jobs?

The code in src/storage is primarily for offline image manipulation,
when no qemu is running and the storage volume is not in use.  For
online manipulation, actions like resize must go through the
virDomainBlockResize(), which is handled in src/qemu/qemu_driver.c as a
call into the qemu QMP monitor command for online resizing.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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