Hi Jay, Thanks for your reply, it helped me get started.
I have been going through the code and some of the sparse docs that are available. This is the code file https://github.com/openstack/nova/blob/master/nova/virt/libvirt/utils.py However I am facing a new issue and require some help. I wanted to modify how openstack handles the cow layer as such and also the qcow2 format. It turns out that openstack issues the external command "qemu-img". First of all, is qemu-img internal to openstack ( I mean code for how qemu-img is implemented is in openstack or in qemu ) It is in openstack, where is the code located. If it is outside openstack, does that mean i have to change the code in qemu and then link those binaries with openstack. Any help would be appreciated. Thanks, Gaurab On Sun, Jul 29, 2012 at 6:35 AM, Jay Pipes <[email protected]> wrote: > On 07/28/2012 11:10 AM, Gaurab Basu wrote: > > Another thing I would like to know is whether it uses snapshot mechanism > > over time. > > What is "it" you are referring to above? Are you asking whether Nova > automatically takes snapshots of images over time? If so, no, it does > not. If a user requests a snapshot of a launched instance, then Nova > will issue snapshot commands -- in the case of the libvirt driver, these > commands would be qemu-img snapshot -c <SNAPSHOT_NAME> <IMAGE_PATH>. > > > I mean how does the copy on write functionality works. Does it keep the > > diff snapshots over time ( or something else ). > > Not sure here whether you are asking how QEMU's copy on write operations > work or whether Nova keeps the base images separate from any VM images. > If you are asking about the latter, the answer is that Nova will create > the virtual machine images by creating a COW image based on the base > image it pulls from Glance -- after making a resized copy of the base > image if it needs to do so to meet the needs of the requested image size > of the VM. > > Snapshots that are taken of virtual machine images on a host are stored > by Nova in Glance. > > > And does the diff work at the level of file or block level? > > AFAIK, CoW and snapshot actions with QEMU are block-level. > > > What is the format that the image is converted to after it is fetched > > from glance. > > There may be no conversion needed at all... it depends on what the > format of the original base image that was stored in Glance. Conversion > between raw/iso and QCOW2 and vice versa is what you see in the code, > and is what is done during migration as Mikal mentioned below. > > > I am fairly new to openstack. > > Can you point me to the specific files in the code where all these > > things are coded. I want to know the details of the > > present state. > > grep for qemu-img in the nova/ directory. You'll see all the files that > call qemu-img commands and then you can go look in those files. > > Best, > -jay > > > Thanks again for your help. > > > > Regards, > > Gaurab > > > > On Sat, Jul 28, 2012 at 11:52 AM, Michael Still > > <[email protected] <mailto:[email protected]>> > wrote: > > > > On 28/07/12 05:42, Gaurab Basu wrote: > > > Hi, > > > > > > I am trying to figure out the technology that openstack uses when > > > multiple VM's having the*same *base image (OS) are provisioned on a > > > physical server. > > > Does it use as many copy as the number of VM's or does it use the > same > > > base image and then copy on write. > > > > > > I need to understand the complete details. Can anybody share some > > > details or point me to some place where I can find the details. > > > > Its pretty hard to provide a complete description of what happens, > > because the code keeps changing. However, assuming you have copy on > > write turned on (which is the default IIRC), and assuming that all of > > the instances have the same disk size, then you end up with: > > > > - the image as fetched from glance, with possible format conversion > > - that image resized to the size the instance requested > > - a cow on write layer for each instance that is using that sized > image > > > > The first should be smallish, the second can be quite large, and the > > third will really depend on how much writing the instances are doing. > > > > Note that this all falls apart if instances are migrated, because as > > part of the migration the copy on write layer is transformed into a > full > > disk image, which is what is shipped over to the new machine. > > > > Hope this helps, > > Mikal > > > > > > > > > > > > _______________________________________________ > > Mailing list: https://launchpad.net/~openstack > > Post to : [email protected] > > Unsubscribe : https://launchpad.net/~openstack > > More help : https://help.launchpad.net/ListHelp > > > > _______________________________________________ > Mailing list: https://launchpad.net/~openstack > Post to : [email protected] > Unsubscribe : https://launchpad.net/~openstack > More help : https://help.launchpad.net/ListHelp >
_______________________________________________ Mailing list: https://launchpad.net/~openstack Post to : [email protected] Unsubscribe : https://launchpad.net/~openstack More help : https://help.launchpad.net/ListHelp
