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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'?