On 08/25/2011 01:24 PM, Yonit Halperin wrote:
Hi all,
here is the original email I sent for #725009.
===> For this plan no change is required in libvirt. <===
This plan does not bring back seamless migration to live. But it does
make the current host switching less noticeable by the user.
The difference from seamless migration is that in-progress operations
fail (equivalent to disconnect and reconnect): audio, smartcard, usb (?
not sure for which rhel version it is designated), vdagent (copy-paste).
After some discussions with Alon, we came to the conclusion that in
order to revive seamless migration we need to solve the following race
conditions :
race 1: target starts before the client managed to connect to it.
possible solutions:
1) qxl savevm will hold (with timeout) migration till client is
connected to target. Will it be acceptable by qemu? (Gerd?)
However, qxl is not necessarily installed...
2) spice will try to connect the target immediately when it receives
client_migrate_info from libvirt, and will wait till it succeeds (and by
this will hold migration. (libvirt - can we count on executing
client_migrate_info just before migration starts and not a long time
before?)
race 2: target starts before it manages to restore the source
pre-migration state (which was sent to it from the src server, via the
client).
solution:
the target won't attach any spice device interface till it restores its
state (with timeout). i.e., it will queue any request from the quest,
and will operate on them only after it has been restored.
From the vm_start code (vl.c), it looks like the start notifier is
called before the guest is actually started, so we can hold the target
from starting till we recover it. Gerd, is this correct?
other changes in spice:
due to the broken seamless migration, spice channels were not maintained
to keep supporting it (i.e., sending the minimal amount of data that is
required for the target to restore the channel's state), especially the
new channels (smartcard, usb(?), agent transmissions, etc.). So,
implementation in this level is required as well.
Arnon, Alon, Hans, can you let me know how much work it would be to
support seemless migration for agent, smartcard, and usb (is it relevant
for rhel 6.3?)? Supporting seemless migration mainly requires (1)
sending SPICE_MSG_MIGRATE_DATA with data that will allow the target side
to be restored, so that the connection with the same client will stay
valid. (2) handle SPICE_MSGC_MIGRATE_DATA in the target
See RedChannel::handle_migrate in the client for more details.
Is there any other channel I didn't think of that needs work for
seemless migration (except for the display channel and surfaces issues)?
Cheers,
Yonit.
On 08/16/2011 12:53 PM, Yonit Halperin wrote:
Hi,
Today, we handle migration in the client side by disconnecting from the
source and connecting to the target. The disconnection
and reconnection occur in response to
SPICE_MSG_MAIN_MIGRATE_SWITCH_HOST, which is sent from the source when
migration ends.
In order to solve RHBZ #725009, i.e., to avoid expiration of tickets at
migration target before spice client has attempted to connect, it has
been proposed that spice client will connect to the target when
migration starts, but will complete the connection only after
migration ends.
This pathway is more similar to how spice worked when we could support
seamless migration. But, in contrast to seamless migration,
we still need to initialize data on the client side upon switching to
the target (this cleanup was achieved by the explicit disconnection and
reconnection).
Here are the details of the implementation I suggest:
qemu
=====
ui/spice-core::migration_state_notifier should handle MIG_STATE_ACTIVE
(for migration start) and MIG_STATE_ERROR/CANCELLED by calling
spice_server_migrate_start, and spice_server_migrate_end, respectively.
These callbacks are currently declared in spice-experimental.h.
spice-server
=============
(A) Migration source side
* reds::spice_server_migrate_start:
send SPICE_MSG_MAIN_MIGRATE_SWITCH_HOST.
We can't use SPICE_MSG_MAIN_MIGRATE_BEGIN since it doesn't
include the certificate information we need. But we can change it
to be identical to SPICE_MSG_MAIN_MIGRATE_SWITCH_HOST.
* reds::spice_server_migrate_end(completed)
- if (completed) => send SPICE_MSG_MIGRATE (flags=0) to all
connected channels (via Channel->migrate).
- if (!completed) => send SPICE_MSG_MAIN_MIGRATE_CANCEL
(B) Migration target side
reds identifies it is a migration target when the client connects with a
connection id != 0.
When linking to a migrated channels, a special treatment is required
(and not the support that is currently coded, since it is for seamless
migration).
For example:
- For the main channel, (1) network test is not required (2) no need for
SPICE_MSG_MAIN_INIT, but rather SPICE_MSG_MAIN_MULTI_MEDIA_TIME and
SPICE_MSG_MAIN_MOUSE_MODE. This way we will also save all the agent work
we preform when initializing the main channel in the client.
- For the display channel, we mustn't call display_channel_wait_for_init
immediately upon link, but we should expect it to arrive later (for
setting cache and dictionary sizes).
- For playback channel: we still need to send the current playback
status, as opposed to seamless migration.
Spice client
============
(A) SPICE_MSG_MAIN_MIGRATE_SWITCH_HOST
client connects to the target, but still stays connected to the
source host. It doesn't listen to the target sockets.
The link message to the target contains the connection_id of the
connection to the source (this allows the target server to identify
itself as a migration target).
For this part we can use most of the code in the class Migrate in
red_client.cpp
(B) SPICE_MSG_MIGRATE
We can use the code in red_channel::handle_migrate to switch the
channels and start listening to the target.
The difference is that we should implement differently the virtual
method RedChannel::on_migrate.
(1) Each channel must reset all the dynamic data that depends on
the server. For example: the display channel
needs to destroy all the surfaces and reset the caches and
dictionary; The playback and record channel need to stop
the current session, if there is an active one, etc.
(2) Each channel should send to the server the initalization
information it normally sends in RedChannel::on_connect.
(C) SPICE_MSG_MAIN_MIGRATE_CANCEL
disconnects all the new channels. This code is already implemented
in spice-client.
spice-protocol(?)/Backward compatibility
=========================================
should we bounce spice protocol version, or use capabilities? (if we
change SPICE_MSG_MAIN_MIGRATE_BEGIN structue, there is no question).
New Spice-Server with old client will send only
SPICE_MSG_MAIN_MIGRATE_SWITCH_HOST, and only when migration completes
(same as today).
New client with old Spice-server will disconnect the source and will
connect the target upon receiving SPICE_MSG_MAIN_MIGRATE_SWITCH_HOST
(same as today).
Any comments? If there are any tricky parts I missed, let me know.
Cheers,
Yonit.
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