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'?
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