On Wed, 08/14 07:29, Spensky, Chad - 0559 - MITLL wrote: > Stefan, Fam, > > We are trying to keep an active shadow copy while the system is running > without any need for pausing. More precisely we want to log every > individual access to the drive into a database so that the entire stream > of accesses could be replayed at a later time. > There's no IO request log infrastructure in QEMU for now. What drive-mirror can do is to repetitively send changed sectors since last sending point, but it's not in guest request order or operation size, it works just with a dirty bitmap. In your methodology you didn't mention the hook you worked in block.c, but I think it is necessary to hack block.c to log every r/w access to the drive, I think you synchronous each write to the shadow image, right? > > > On 8/14/13 6:05 AM, "Stefan Hajnoczi" <stefanha@xxxxxxxxx> wrote: > > >On Wed, Aug 14, 2013 at 10:40:06AM +0800, Fam Zheng wrote: > >> On Tue, 08/13 16:13, Spensky, Chad - 0559 - MITLL wrote: > >> > Hi All, > >> > > >> > I'm working with some disk introspection on KVM, and we trying to > >>create > >> > a shadow image of the disk. We've hooked the functions in block.c, in > >> > particular bdrv_aio_writev. However we are seeing writes go through, > >> > pausing the VM, and the comparing our shadow image with the actual VM > >> > image, and they aren't 100% synced up. The first 1-2 sectors appear > >>to be > >> > always be correct, however, after that, there are sometimes some > >> > discrepancies. I believe we have exhausted most obvious bugs (malloc > >> > bugs, incorrect size calculations etc.). Has anyone had any > >>experience > >> > with this or have any insights? > >> > > >> > Our methodology is as follows: > >> > 1. Boot the VM. > >> > 2. Pause VM. > >> > 3. Copy the disk to our shadow image. > >> > >> How do you copy the disk, from guest or host? > >> > >> > 4. Perform very few reads/writes. > >> > >> Did you flush to disk? > >> > >> > 5. Pause VM. > >> > 6. Compare shadow copy with active vm disk. > >> > > >> > And this is where we are seeing discrepancies. Any help is much > >> > appreciated! We are running on Ubuntu 12.04 with a modified Debian > >>build. > >> > > >> > - Chad > >> > > >> > -- > >> > Chad S. Spensky > >> > > >> > >> I think drive-backup command does just what you want, it creates a image > >> and copy-on-write date from guest disk to the target, without pausing > >> VM. > > > >Or perhaps drive-mirror. > > > >Maybe Chad can explain what the use case is. There is probably an > >existing command that does this or that could be extended to do this > >safely. > > > >Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
