From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: live saving of domU Date: Wed, 10 May 2006 11:59:09 -0500 Message-ID: <44621BDD.9060401@us.ibm.com> References: <871640aa0605100840t14a9f9deqf5c9be99f56917b7@mail.gmail.com> <20060510163213.GF4954@leeni.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20060510163213.GF4954@leeni.uk.xensource.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Ewan Mellor Cc: xen-devel , Jayesh Salvi List-Id: xen-devel@lists.xenproject.org Ewan Mellor wrote: > On Wed, May 10, 2006 at 10:40:31AM -0500, Jayesh Salvi wrote: > > >> Hi, >> >> Could anyone tell me, why 'xm save' has live parameter set to false by >> default. From yesterday's patch ([PATCH] [XenD] Migration-related change) >> i guess this paramter is renamed to network. >> > > This parameter has not been renamed -- the rename was for a similar flag > passed in to the device migration code, but the live flag for migration > remains. > > >> [Snip] >> >> I am interested in saving the state of a virtual machine to a file, but >> want to continue it running. I want to backup the state of the machine, so >> I want this to be unintrusive operation. I would like to pause the domU >> and save it to file but keeping it still in memory. After save to file is >> done I will unpause the domU. >> >> I don't see why this shouldn't be possible if live migration works so >> well. >> > > The reason it's not supported at the moment is this: if you take a snapshot of > a VM, then run for a bit, and then try and run the snapshot against the same > filesystem that you were using before, you will inevitably corrupt the > filesystem. > Moreover, you cannot dump the state of a domain after a pause and expect it to ever run again. Guests are aware of the physical addresses of the memory that's been allocated to them. Because of this, to save a domain's state in a restorable way you need the guest to "canonicalize" itself. The only way to do this today is through a suspend operation which happens to be a subop of shutdown. Shutdowns are non-recoverable so you cannot use this as a snapshotting mechanism. The closest thing you can achieve is a localhost migration. There are some caveats to this, of course. The first is that you need to have as much memory as the domain has available since you'll have a copy of the domain created briefly while the migration takes place. Migrations are also quite intrusive since they involve tearing down and bringing up all the devices. I've gotten a lot of requests for light weight checkpointing. AFAIK, noone is actually working on it though. Regards, Anthony Liguori > If you had a way to snapshot the storage at the same time as the VM, then you > could make live snapshotting of VMs work properly. As it is, this is not > integrated into Xend at the moment, and not supported because of the danger of > filesystem corruption if you don't know what you are doing. > > Ewan. > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel >