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

[Greg Padgett]

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

> Advance thanks,
> Satheesaran S
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