It seems like the following commit broke live migrations of instances with vfat config drive -- https://review.openstack.org/561704. Reproduced the problem on pike with that patch backport, and I think master is affected as well.

The exception happening is:

May 10 17:01:48 cmp02 nova-compute[4443]: Traceback (most recent call last):
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/eventlet/hubs/poll.py", line 115, in wait
May 10 17:01:48 cmp02 nova-compute[4443]:     listener.cb(fileno)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/eventlet/greenthread.py", line 214, in main
May 10 17:01:48 cmp02 nova-compute[4443]:     result = function(*args, **kwargs)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/nova/utils.py", line 1004, in context_wrapper
May 10 17:01:48 cmp02 nova-compute[4443]:     return func(*args, **kwargs)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/nova/virt/libvirt/driver.py", line 6568, in _live_migration_operation
May 10 17:01:48 cmp02 nova-compute[4443]:     LOG.error("Live Migration failure: %s", e, instance=instance)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/oslo_utils/excutils.py", line 220, in __exit__
May 10 17:01:48 cmp02 nova-compute[4443]:     self.force_reraise()
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/oslo_utils/excutils.py", line 196, in force_reraise
May 10 17:01:48 cmp02 nova-compute[4443]:     six.reraise(self.type_, self.value, self.tb)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/nova/virt/libvirt/driver.py", line 6562, in _live_migration_operation
May 10 17:01:48 cmp02 nova-compute[4443]:     bandwidth=CONF.libvirt.live_migration_bandwidth)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/nova/virt/libvirt/guest.py", line 670, in migrate
May 10 17:01:48 cmp02 nova-compute[4443]:     destination, params=params, flags=flags)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/eventlet/tpool.py", line 186, in doit
May 10 17:01:48 cmp02 nova-compute[4443]:     result = proxy_call(self._autowrap, f, *args, **kwargs)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/eventlet/tpool.py", line 144, in proxy_call
May 10 17:01:48 cmp02 nova-compute[4443]:     rv = execute(f, *args, **kwargs)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/eventlet/tpool.py", line 125, in execute
May 10 17:01:48 cmp02 nova-compute[4443]:     six.reraise(c, e, tb)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/eventlet/tpool.py", line 83, in tworker
May 10 17:01:48 cmp02 nova-compute[4443]:     rv = meth(*args, **kwargs)
May 10 17:01:48 cmp02 nova-compute[4443]:   File "/usr/lib/python2.7/dist-packages/libvirt.py", line 1702, in migrateToURI3
May 10 17:01:48 cmp02 nova-compute[4443]:     if ret == -1: raise libvirtError ('virDomainMigrateToURI3() failed', dom=self)
May 10 17:01:48 cmp02 nova-compute[4443]: libvirtError: operation failed: migration of disk vdb failed: Input/output error
May 10 17:01:48 cmp02 nova-compute[4443]: Removing descriptor: 29

(vdb is a vfat config drive)

after turning on the info logging in qemu seen the following in the syslog:

May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.204+0000: 10323: info : qemuMonitorSend:1076 : QEMU_MONITOR_SEND_MSG: mon=0x7f6fe801b920 msg={"execute":"drive-mirror","arguments":{"device":"drive-virtio-disk0","target":"nbd:10.60.0.105:49153:exportname=drive-virtio-disk0","speed":9223372036853727232,"sync":"top","mode":"existing","format":"raw"},"id":"libvirt-208"}
May 10 17:01:48 cmp02 libvirtd[10318]:  fd=-1
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.204+0000: 10318: info : qemuMonitorIOWrite:552 : QEMU_MONITOR_IO_WRITE: mon=0x7f6fe801b920 buf={"execute":"drive-mirror","arguments":{"device":"drive-virtio-disk0","target":"nbd:10.60.0.105:49153:exportname=drive-virtio-disk0","speed":9223372036853727232,"sync":"top","mode":"existing","format":"raw"},"id":"libvirt-208"}
May 10 17:01:48 cmp02 libvirtd[10318]:  len=228 ret=228 errno=0
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.296+0000: 10323: info : qemuMonitorSend:1076 : QEMU_MONITOR_SEND_MSG: mon=0x7f6fe801b920 msg={"execute":"drive-mirror","arguments":{"device":"drive-virtio-disk1","target":"nbd:10.60.0.105:49153:exportname=drive-virtio-disk1","speed":9223372036853727232,"sync":"top","mode":"existing","format":"raw"},"id":"libvirt-209"}
May 10 17:01:48 cmp02 libvirtd[10318]:  fd=-1
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.296+0000: 10318: info : qemuMonitorIOWrite:552 : QEMU_MONITOR_IO_WRITE: mon=0x7f6fe801b920 buf={"execute":"drive-mirror","arguments":{"device":"drive-virtio-disk1","target":"nbd:10.60.0.105:49153:exportname=drive-virtio-disk1","speed":9223372036853727232,"sync":"top","mode":"existing","format":"raw"},"id":"libvirt-209"}
May 10 17:01:48 cmp02 libvirtd[10318]:  len=228 ret=228 errno=0
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.388+0000: 10318: info : qemuMonitorJSONIOProcessLine:208 : QEMU_MONITOR_RECV_EVENT: mon=0x7f6fe801b920 event={"timestamp": {"seconds": 1525971708, "microseconds": 387891}, "event": "BLOCK_JOB_ERROR", "data": {"device": "drive-virtio-disk1", "operation": "write", "action": "report"}}
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.388+0000: 10318: info : qemuMonitorJSONIOProcessLine:208 : QEMU_MONITOR_RECV_EVENT: mon=0x7f6fe801b920 event={"timestamp": {"seconds": 1525971708, "microseconds": 388730}, "event": "BLOCK_JOB_READY", "data": {"device": "drive-virtio-disk0", "len": 1507328, "offset": 1507328, "speed": 9223372036853727232, "type": "mirror"}}
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.392+0000: 10318: info : qemuMonitorJSONIOProcessLine:208 : QEMU_MONITOR_RECV_EVENT: mon=0x7f6fe801b920 event={"timestamp": {"seconds": 1525971708, "microseconds": 392754}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "drive-virtio-disk1", "len": 67108864, "offset": 196608, "speed": 9223372036853727232, "type": "mirror", "error": "Input/output error"}}
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.393+0000: 10318: info : qemuMonitorJSONIOProcessLine:213 : QEMU_MONITOR_RECV_REPLY: mon=0x7f6fe801b920 reply={"return": [{"io-status": "ok", "device": "drive-virtio-disk0", "busy": false, "len": 1507328, "offset": 1507328, "paused": false, "speed": 9223372036853727232, "ready": true, "type": "mirror"}], "id": "libvirt-210"}
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.395+0000: 10323: error : qemuMigrationDriveMirrorReady:624 : operation failed: migration of disk vdb failed: Input/output error
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.395+0000: 10323: info : qemuMonitorSend:1076 : QEMU_MONITOR_SEND_MSG: mon=0x7f6fe801b920 msg={"execute":"block-job-cancel","arguments":{"device":"drive-virtio-disk0"},"id":"libvirt-211"}
May 10 17:01:48 cmp02 libvirtd[10318]:  fd=-1
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.395+0000: 10318: info : qemuMonitorIOWrite:552 : QEMU_MONITOR_IO_WRITE: mon=0x7f6fe801b920 buf={"execute":"block-job-cancel","arguments":{"device":"drive-virtio-disk0"},"id":"libvirt-211"}
May 10 17:01:48 cmp02 libvirtd[10318]:  len=95 ret=95 errno=0
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.398+0000: 10323: info : qemuMonitorSend:1076 : QEMU_MONITOR_SEND_MSG: mon=0x7f6fe801b920 msg={"execute":"block-job-cancel","arguments":{"device":"drive-virtio-disk1"},"id":"libvirt-212"}
May 10 17:01:48 cmp02 libvirtd[10318]:  fd=-1
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.398+0000: 10318: info : qemuMonitorIOWrite:552 : QEMU_MONITOR_IO_WRITE: mon=0x7f6fe801b920 buf={"execute":"block-job-cancel","arguments":{"device":"drive-virtio-disk1"},"id":"libvirt-212"}
May 10 17:01:48 cmp02 libvirtd[10318]:  len=95 ret=95 errno=0
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.402+0000: 10318: info : qemuMonitorJSONIOProcessLine:208 : QEMU_MONITOR_RECV_EVENT: mon=0x7f6fe801b920 event={"timestamp": {"seconds": 1525971708, "microseconds": 401820}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "drive-virtio-disk0", "len": 1507328, "offset": 1507328, "speed": 9223372036853727232, "type": "mirror"}}
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.403+0000: 10318: info : qemuMonitorJSONIOProcessLine:213 : QEMU_MONITOR_RECV_REPLY: mon=0x7f6fe801b920 reply={"id": "libvirt-212", "error": {"class": "DeviceNotActive", "desc": "Block job 'drive-virtio-disk1' not found"}}
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.414+0000: 10318: info : qemuMonitorJSONIOProcessLine:213 : QEMU_MONITOR_RECV_REPLY: mon=0x7f6fe801b920 reply={"return": [{"device": "drive-virtio-disk0", "parent": {"stats": {"flush_total_time_ns": 0, "wr_highest_offset": 1835008, "wr_total_time_ns": 0, "failed_wr_operations": 0, "failed_rd_operations": 0, "wr_merged": 0, "wr_bytes": 0, "timed_stats": [], "failed_flush_operations": 0, "account_invalid": false, "rd_total_time_ns": 0, "flush_operations": 0, "wr_operations": 0, "rd_merged": 0, "rd_bytes": 0, "invalid_flush_operations": 0, "account_failed": false, "rd_operations": 0, "invalid_wr_operations": 0, "invalid_rd_operations": 0}, "node-name": "#block002"}, "stats": {"flush_total_time_ns": 9890804, "wr_highest_offset": 32911872, "wr_total_time_ns": 274684321, "failed_wr_operations": 0, "failed_rd_operations": 0, "wr_merged": 5, "wr_bytes": 268288, "timed_stats": [], "failed_flush_operations": 0, "account_invalid": true, "rd_total_time_ns": 182145276, "flush_operations": 22, "wr_operations": 87, "rd_merged": 7, "rd_bytes": 20611072, "invalid_flush_operations": 0, "account_failed": true, "idle_time_ns": 783247565442, "rd_operations": 912, "invalid_wr_operations": 0, "invalid_rd_operations": 0}, "backing": {"parent": {"stats": {"flush_total_time_ns": 0, "wr_highest_offset": 0, "wr_total_time_ns": 0, "failed_wr_operations": 0, "failed_rd_operations": 0, "wr_merged": 0, "wr_bytes": 0, "timed_stats": [], "failed_flush_operations": 0, "account_invalid": false, "rd_total_time_ns": 0, "flush_operations": 0, "wr_operations": 0, "rd_merged": 0, "rd_bytes": 0, "invalid_flush_operations": 0, "account_failed": false, "rd_operations": 0, "invalid_wr_operations": 0, "invalid_rd_operations": 0}, "node-name": "#block264"}, "stats": {"flush_total_time_ns": 0, "wr_highest_offset": 0, "wr_total_time_ns": 0, "failed_wr_operations": 0, "failed_rd_operations": 0, "wr_merged": 0, "wr_bytes": 0, "timed_stats": [], "failed_flush_operations": 0, "account_invalid": false, "rd_total_time_ns": 0, "flush_operations": 0,
May 10 17:01:48 cmp02 libvirtd[10318]:  "wr_operations": 0, "rd_merged": 0, "rd_bytes": 0, "invalid_flush_operations": 0, "account_failed": false, "rd_operations": 0, "invalid_wr_operations": 0, "invalid_rd_operations": 0}, "node-name": "#block392"}, "node-name": "#block108"}, {"device": "drive-virtio-disk1", "parent": {"stats": {"flush_total_time_ns": 0, "wr_highest_offset": 0, "wr_total_time_ns": 0, "failed_wr_operations": 0, "failed_rd_operations": 0, "wr_merged": 0, "wr_bytes": 0, "timed_stats": [], "failed_flush_operations": 0, "account_invalid": false, "rd_total_time_ns": 0, "flush_operations": 0, "wr_operations": 0, "rd_merged": 0, "rd_bytes": 0, "invalid_flush_operations": 0, "account_failed": false, "rd_operations": 0, "invalid_wr_operations": 0, "invalid_rd_operations": 0}, "node-name": "#block458"}, "stats": {"flush_total_time_ns": 0, "wr_highest_offset": 0, "wr_total_time_ns": 0, "failed_wr_operations": 0, "failed_rd_operations": 0, "wr_merged": 0, "wr_bytes": 0, "timed_stats": [], "failed_flush_operations": 0, "account_invalid": true, "rd_total_time_ns": 50271508, "flush_operations": 0, "wr_operations": 0, "rd_merged": 8, "rd_bytes": 1889792, "invalid_flush_operations": 0, "account_failed": true, "idle_time_ns": 2412792465348, "rd_operations": 359, "invalid_wr_operations": 0, "invalid_rd_operations": 0}, "node-name": "#block543"}], "id": "libvirt-214"}
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.823+0000: 10323: error : virNetClientProgramDispatchError:177 : migration successfully aborted
May 10 17:01:48 cmp02 libvirtd[10318]: 2018-05-10 17:01:48.845+0000: 10318: error : virNetSocketReadWire:1811 : End of file while reading data: Input/output error

It seems like qemu's drive mirror expects the full disk size to be present on dest compute, not only what is allocated.

Revert of the patch solved the issue.