[libvirt-users] Reg: content of disk is not reflecting in host.
Hi Team, I am doing a small testing and I don't know if my expectation is correct or not. Pardon me if I am ignorant. I created a VM and the VM is running. In the hypervisor I have created ".img" file and attached this .img file to the VM. My expectation is that, if VM is writing files to the attached disk, then it should reflect in the .img file created in the hypervisor. But It is not working as my expectation. Please correct me if my expectation is wrong. Steps: 1. Created disk.img in the hypervisor using the command: dd if=/dev/zero of=disk.img bs=1M count=50; mkfs ext3 -F disk.img 2. Attached the disk to the running VM using the command: virsh attach-disk <Domain-Name> --source disk.img --target vdb --live 3. In the VM, I mounted the disk and created few files. 4. In the hypervisor, I mounted the disk.img to check if the file created in the VM exists in the .img file.
I am not able to see those files.
Regards, Bharath
On Wed, Aug 07, 2019 at 09:57:02PM +0530, bharath paulraj wrote:
Hi Team,
I am doing a small testing and I don't know if my expectation is correct or not. Pardon me if I am ignorant. I created a VM and the VM is running. In the hypervisor I have created ".img" file and attached this .img file to the VM. My expectation is that, if VM is writing files to the attached disk, then it should reflect in the .img file created in the hypervisor. But It is not working as my expectation. Please correct me if my expectation is wrong.
Steps: 1. Created disk.img in the hypervisor using the command: dd if=/dev/zero of=disk.img bs=1M count=50; mkfs ext3 -F disk.img 2. Attached the disk to the running VM using the command: virsh attach-disk <Domain-Name> --source disk.img --target vdb --live 3. In the VM, I mounted the disk and created few files. 4. In the hypervisor, I mounted the disk.img to check if the file created in the VM exists in the .img file.
I am not able to see those files.
Do *NOT* do step 4 - it is incredibly dangerous in general and may well result in filesystem corruption & serious data loss. Most filesystems are only designed to be used by a single OS at any time. By mounting it on the host, you have 2 separate OS both accessing the same filesystem. Even if you mount a filesystem with the read-only flag, there can still be writes to the filesystem as the second OS to mount will see it "dirty" and try to replay the journal that reflects what the first OS was doing. Assuming you didn't actually corrupt your FS image, the likely reason you don't see the file from the host is that the guest OS probably hasn't flushed it out to disk yet - it'll still be cached in memory in the guest unless something explicitly ran 'sync'. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
Thank you for the clarification. Regards, Bharath On Wed, Aug 7, 2019 at 10:16 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
On Wed, Aug 07, 2019 at 09:57:02PM +0530, bharath paulraj wrote:
Hi Team,
I am doing a small testing and I don't know if my expectation is correct or not. Pardon me if I am ignorant. I created a VM and the VM is running. In the hypervisor I have created ".img" file and attached this .img file to the VM. My expectation is that, if VM is writing files to the attached disk, then it should reflect in the .img file created in the hypervisor. But It is not working as my expectation. Please correct me if my expectation is wrong.
Steps: 1. Created disk.img in the hypervisor using the command: dd if=/dev/zero of=disk.img bs=1M count=50; mkfs ext3 -F disk.img 2. Attached the disk to the running VM using the command: virsh attach-disk <Domain-Name> --source disk.img --target vdb --live 3. In the VM, I mounted the disk and created few files. 4. In the hypervisor, I mounted the disk.img to check if the file created in the VM exists in the .img file.
I am not able to see those files.
Do *NOT* do step 4 - it is incredibly dangerous in general and may well result in filesystem corruption & serious data loss.
Most filesystems are only designed to be used by a single OS at any time.
By mounting it on the host, you have 2 separate OS both accessing the same filesystem. Even if you mount a filesystem with the read-only flag, there can still be writes to the filesystem as the second OS to mount will see it "dirty" and try to replay the journal that reflects what the first OS was doing.
Assuming you didn't actually corrupt your FS image, the likely reason you don't see the file from the host is that the guest OS probably hasn't flushed it out to disk yet - it'll still be cached in memory in the guest unless something explicitly ran 'sync'.
Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
-- Regards, Bharath
participants (2)
-
bharath paulraj -
Daniel P. Berrangé