[ovirt-users] VMs seems to be slow, while snapshot delete is happening

SATHEESARAN sasundar at redhat.com
Tue May 17 03:37:51 UTC 2016

On 05/14/2016 02:38 AM, Greg Padgett wrote:
> On 05/12/2016 03:46 AM, SATHEESARAN wrote:
>> Hi All,
>> I have created a VM with 60GB of disk space.
>> I have started I/O inside the VM, after sometime when the disk was 
>> 4GB full,
>> I took a snapshot of the disk from UI.
>> I/O kept continuing and the new image file ( overlay file ) was 15GB in
>> size.
>> Now I deleted the snapshot.
>> I think this would initiate block-merges.
>> During this time, my I/O was were slower, till the snapshot delete is
>> completed.
>> Its not very slow, but slow.
>> Is this a known behavior ?
> Hi Satheesaran,
> It sounds like this was a live merge, ie snapshot removal while the VM 
> was running.  This is currently implemented as a block commit, where 
> data from the snapshot (your 15GB overlay, in this case) is merged down.
> For snapshot deletion to complete, this means that your concurrent I/O 
> and the data already in the overlay have to be written into the 
> next-lower image.  So in short, yes, depending on the storage system 
> backing your VM, I/O may slow down a bit until completion.
> HTH,
> Greg

Thanks for that information.
I too expected it, but really wanted to confirm the behavior.

-- Satheesaran S

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