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If anyone curious about this bug: It was clearly linux-3.13  bug, issue 
was completely solved by moving to 4.4.

On 02/15/2017 08:03 PM, George Shuklin wrote:
> Hello.
>
> We've upgraded to mitaka, qemu 2.5/linux-3.13 and found that raw 
> images now have BIG issues with snapshots.
>
> Symptoms:
>
> When snapshot process reach 'fallocated' blocks (see below) all IO in 
> guest start lagging, including network IO. Windows starts loosing 
> pings for very long period of time (~30-40 minutes), linux do this 
> briefly (~500-700ms, but regularly).
>
> Research so far:
>
> All those symptoms may be resolved if every disk block is actually 
> written on disk (dd if=disk of=disk conv=notrunc). If file has 
> fallocated blocks, it will cause problem. If it has sparse hole it 
> will cause problem too, but when preallocate_images = space, no any 
> sparse holes are in the file.
>
> Best way so far to distinguish 'bad' disk from 'good' is to use 
> filefrag -v. For 'bad' disk it shows "unwritten" flag.
>
> 1. Any idea how to prevent this?
>
> 2. Any idea how to force nova to actually write images completely 
> without using 'fallocate'?
>