Freezing shadows helping data backup

Introducing initial commit of new feature: blade image freeze with shadowing support. What it means? Its now possible to stop any modifications to disk image file or device, while keeping client to work as usual (may be only a bit slower than usual). How it works? When received 'freeze' signal AoEde flushes all its internal buffers and begin functioning in special freezed mode until gets 'unfreeze' signal. Also you can specify temporary shadow file, where all writes will go instead while actual image remains 'freeze'-d. Use option -f for that, like:
aoede -b 64 1 1 egiga0 /var/.AOE/disk11.fs -f /var/.AOE/shadow11.fs
Also make sure that shadow file located on filesystem that supports sparse files, otherwise it will instantly occupy a bit more space than size of main image.
How to freeze/unfreeze? Very simple. Use kill for that:
freeze:
kill -USR1 ${AOEDE_PID}
kill -USR2 ${AOEDE_PID}
unfreeze:
kill -USR1 ${AOEDE_PID}
kill -USR1 ${AOEDE_PID}
kill -USR2 ${AOEDE_PID}
Note that freezing/and unfreezing doesn't performed instantly. So after sending freeze sequence wait until AoEde will create shadow file. And after sending unfreeze - wait until that file will be removed. KISS as it is.
Unfreezing flushes written data to main image 'in background', without stopping serving initiator requests. However this process noticeable decreases performance until finishes. Nothing comes free in this world :(