Great description. Thanks George. On 28 February 2012 18:03, George Mihaiescu <george.mihaie...@q9.com> wrote: > When you ask Nova to boot a VM, nova-compute will connect to Glance and "GET" > the image file from Glance and save it on the its local filesystem in > "/var/lib/nova/instances/_base". > > If Glance is set to use Swift as its backend storage, then Glance will get > that file from Swift (through the Proxy). If not, then it will stream the > file from Glance's filesystem (check the variable "filesystem_store_datadir" > in the file "glance-api.conf" to see what Glance is set to use as backend > store). > > So by default the disk of an instance is basically stored on the local > filesystem of the server where the instance is running (in > "/var/lib/nova/instances/instance-0000000X/disk"), and it's called ephemeral > because when you terminate the instance the entire directory > "/var/lib/nova/instances/instance-0000000X" gets deleted and the virtual disk > is gone, but the base image in the "_base" directory is not touched. > > If the virtual disk is using qcow2 then only the changes that occur from the > baseline are captured in the virtual disk, so the disk grows as the instance > is changed more. The benefit is that you can have five instances using the > same base template without using five times the space on the local filesystem > (read http://people.gnome.org/~markmc/qcow-image-format.html for more info > about qcow2). > > Persistent volumes are virtual disks that you attach to a running instance > using the nova-volume service. These virtual disks are actually LVM volumes > exported over iSCSI by the nova-volume server. They are called persistent > because they are not affected by an instance being terminated, or by a > nova-compute server crashing. You could just start a new instance and > re-attach that volume and get your data back. The nova-volume is using LVM + > iSCSI but there are drivers/plugins for Nexenta (and Netapp will release its > own soon), so there are enterprise grade options available. > > I hope this helps you understand this topic (and I hope I'm right in my > explanation). > > George > > > -----Original Message----- > From: openstack-bounces+george.mihaiescu=q9....@lists.launchpad.net > [mailto:openstack-bounces+george.mihaiescu=q9....@lists.launchpad.net] On > Behalf Of Michaël Van de Borne > Sent: Tuesday, February 28, 2012 3:52 AM > To: openstack@lists.launchpad.net > Subject: Re: [Openstack] understanding ephemeral and persistant volumes > > is it too complicated or too simple to be answered on the ML? > > > Le 23/02/12 16:46, Michaël Van de Borne a écrit : >> Hi all, >> >> I'd like to understand how things go with ephemeral and persistant >> volumes. >> For instance, say that my gold images are stored in a Swift storage >> network, connected to Glance. >> >> When I ask Nova to boot the VM, >> - will the disk image stay in Swift storage? >> - will the physical compute node copy the image from Swift to its >> local filesystem? >> - will ephemeral volumes be stored on local compute node filesystem >> whereas persistant drives be stored in Swift? >> >> According to these answers, I'll know if the compute nodes of my cloud >> should have disks attached or if no data will ever be stored on these >> nodes even when VMs are running. >> >> maybe this is documented somewhere, but I didn't find clear >> information about ephemeral and persistant volume management? >> >> thank you, >> >> Michaël >> >> > > _______________________________________________ > Mailing list: https://launchpad.net/~openstack > Post to : openstack@lists.launchpad.net > Unsubscribe : https://launchpad.net/~openstack > More help : https://help.launchpad.net/ListHelp > > _______________________________________________ > Mailing list: https://launchpad.net/~openstack > Post to : openstack@lists.launchpad.net > Unsubscribe : https://launchpad.net/~openstack > More help : https://help.launchpad.net/ListHelp
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