Re: [libvirt] [Qemu-devel] [PATCH v4] XBZRLE delta for live migration of large memory apps
On 08/10/2011 11:40 AM, Avi Kivity wrote:
On 08/10/2011 07:23 PM, Anthony Liguori wrote:
Right now we have capabilties in the form of -help output.
If -help says
-no-xzbrle disable xzbrle support
(or -migration-compression xzbrle=off, or something) that's sufficient for management tools.
This is static, not dynamic. You may attempt to migrate to another host that supports it and then migrate to a second host that doesn't support it after the first migration fails.
This may be acceptable, wait until the entire migration cluster is xzbrle capable before enabling it. If not, add a monitor command.
1) xzbrle needs to be disabled by default. That way management tools don't unknowingly enable it by not passing -no-xzbrle. 2) there needs to be a mechanism for the management tool to query whether qemu supports xzbrle. 3) a management tool should be able to query the source and destination, and then enable xzbrle if both sides support it. You can argue that (3) could be static. A command could be added to toggle it dynamically through the monitor. But no matter what, someone has to touch libvirt and any other tool that works with QEMU to make this thing work. But this is a general problem. Any optional change to the migration protocol has exactly the same characteristics whether it's XZBRLE, XZBRLE v2 (if there is a v2), ASN.1, or any other form of compression that rolls around. Instead of teaching management tools how to deal with all of these things, let's just fix this problem once. It just takes: a) A query-migration-caps command that returns a dict with two lists of strings. Something like: { 'execute': 'query-migration-caps' } { 'return' : { 'capabilities': [ 'xbzrle' ], 'current': [] } } b) A set-migration-caps command that takes a list of strings. It simply takes the intersection of the capabilities set with the argument and sets the current set to the result. Something like: { 'execute': 'set-migration-caps', 'arguments': { 'set': [ 'xbzrle' ] }} { 'return' : {} } c) An internal interface to register a capability and an internal interface to check if a capability is currently enabled. The xzbrle code just needs to disable itself if the capability isn't set. Then we teach libvirt (and other tools) to query the caps list on the source, set the destination, query the current set on the destination, and then set that set on the source. As we introduce new things, like the next great compression protocol, or ASN.1, we don't need to touch libvirt again. libvirt can still know about the caps and selectively override QEMU if it's so inclined but it prevents us from reinventing the same mechanisms over and over again.
We shouldn't block this feature just because some monitor facility is not yet implemented.
We shouldn't make *any* changes to the migration protocol before we have a feature negotiation capability. I only want to do a hard break of the protocol once.
Didn't we agree that management tool mediated feature negotiation (that is, outside the migration protocol itself) is acceptable?
Yes. But that negotiation needs to become part of the "protocol" for migration. In the absence of that negotiation, we need to use the wire protocol we use today. We cannot have ad-hoc feature negotiation for every change we make to the wire protocol. Regards, Anthony Liguori
From: Anthony Liguori [mailto:anthony@codemonkey.ws] Sent: Wednesday, August 10, 2011 10:28 PM To: Avi Kivity Cc: Blue Swirl; Stefan Hajnoczi; Shribman, Aidan; qemu-devel Developers; libvir-list@redhat.com Subject: Re: [Qemu-devel] [PATCH v4] XBZRLE delta for live migration of large memory apps
a) A query-migration-caps command that returns a dict with two lists of strings. Something like:
{ 'execute': 'query-migration-caps' } { 'return' : { 'capabilities': [ 'xbzrle' ], 'current': [] } }
b) A set-migration-caps command that takes a list of strings. It simply takes the intersection of the capabilities set with the argument and sets the current set to the result. Something like:
{ 'execute': 'set-migration-caps', 'arguments': { 'set': [ 'xbzrle' ] }} { 'return' : {} }
We may want to further sub-divide capabilities into categories: { 'execute': 'query-migration-caps' } { 'return' : { 'encoding' : { 'current', 'asn.1', 'proto2', 'thrift', etc. } }, { 'delta' : { 'xbzrle', "xdelta", ...} }, { 'compression' : { 'snappy', 'lzo' } } } This would help libvirt/management to select features automatically or manually (via UI) without having to 'understand' the any given capability meaning.
Yes. But that negotiation needs to become part of the "protocol" for migration. In the absence of that negotiation, we need to use the wire protocol we use today. We cannot have ad-hoc feature negotiation for every change we make to the wire protocol.
Agreed. Therefore caps plus xbzrle could be added before ASN.1/v1.0 without breaking anything as long as when 'set-migration-caps' is not issued Qemu uses the current protocol. Aidan
On 08/11/2011 03:03 AM, Shribman, Aidan wrote:
From: Anthony Liguori [mailto:anthony@codemonkey.ws] Sent: Wednesday, August 10, 2011 10:28 PM To: Avi Kivity Cc: Blue Swirl; Stefan Hajnoczi; Shribman, Aidan; qemu-devel Developers; libvir-list@redhat.com Subject: Re: [Qemu-devel] [PATCH v4] XBZRLE delta for live migration of large memory apps
a) A query-migration-caps command that returns a dict with two lists of strings. Something like:
{ 'execute': 'query-migration-caps' } { 'return' : { 'capabilities': [ 'xbzrle' ], 'current': [] } }
b) A set-migration-caps command that takes a list of strings. It simply takes the intersection of the capabilities set with the argument and sets the current set to the result. Something like:
{ 'execute': 'set-migration-caps', 'arguments': { 'set': [ 'xbzrle' ] }} { 'return' : {} }
We may want to further sub-divide capabilities into categories: { 'execute': 'query-migration-caps' } { 'return' : { 'encoding' : { 'current', 'asn.1', 'proto2', 'thrift', etc. } }, { 'delta' : { 'xbzrle', "xdelta", ...} }, { 'compression' : { 'snappy', 'lzo' } } } This would help libvirt/management to select features automatically or manually (via UI) without having to 'understand' the any given capability meaning.
I would prefer caps to be mostly transparent to libvirt. In fact, I'd like to see exactly three caps: xbzrle, asn1, and autonegotiate. I'd like to move the caps negotation into the protocol itself.
Yes. But that negotiation needs to become part of the "protocol" for migration. In the absence of that negotiation, we need to use the wire protocol we use today. We cannot have ad-hoc feature negotiation for every change we make to the wire protocol.
Agreed. Therefore caps plus xbzrle could be added before ASN.1/v1.0 without breaking anything as long as when 'set-migration-caps' is not issued Qemu uses the current protocol.
Exactly. Regards, Anthony Liguori
Aidan
On 08/10/2011 10:27 PM, Anthony Liguori wrote:
This may be acceptable, wait until the entire migration cluster is xzbrle capable before enabling it. If not, add a monitor command.
1) xzbrle needs to be disabled by default. That way management tools don't unknowingly enable it by not passing -no-xzbrle.
We could hook it to -M, though it's a bit gross. Otherwise we need to document this clearly in the management tool author's guide.
3) a management tool should be able to query the source and destination, and then enable xzbrle if both sides support it.
You can argue that (3) could be static. A command could be added to toggle it dynamically through the monitor.
But no matter what, someone has to touch libvirt and any other tool that works with QEMU to make this thing work. But this is a general problem. Any optional change to the migration protocol has exactly the same characteristics whether it's XZBRLE, XZBRLE v2 (if there is a v2), ASN.1, or any other form of compression that rolls around.
If we have two-way communication we can do this transparently in the protocol itself.
Instead of teaching management tools how to deal with all of these things, let's just fix this problem once. It just takes:
a) A query-migration-caps command that returns a dict with two lists of strings. Something like:
{ 'execute': 'query-migration-caps' } { 'return' : { 'capabilities': [ 'xbzrle' ], 'current': [] } }
b) A set-migration-caps command that takes a list of strings. It simply takes the intersection of the capabilities set with the argument and sets the current set to the result. Something like:
{ 'execute': 'set-migration-caps', 'arguments': { 'set': [ 'xbzrle' ] }} { 'return' : {} }
c) An internal interface to register a capability and an internal interface to check if a capability is currently enabled. The xzbrle code just needs to disable itself if the capability isn't set.
Then we teach libvirt (and other tools) to query the caps list on the source, set the destination, query the current set on the destination, and then set that set on the source.
This is only if the capability has no side effect.
As we introduce new things, like the next great compression protocol, or ASN.1, we don't need to touch libvirt again. libvirt can still know about the caps and selectively override QEMU if it's so inclined but it prevents us from reinventing the same mechanisms over and over again.
Right.
Yes. But that negotiation needs to become part of the "protocol" for migration. In the absence of that negotiation, we need to use the wire protocol we use today. We cannot have ad-hoc feature negotiation for every change we make to the wire protocol.
Okay, as long as we have someone willing to implement it. -- error compiling committee.c: too many arguments to function
On Thu, Aug 11, 2011 at 11:17:09AM +0300, Avi Kivity wrote:
On 08/10/2011 10:27 PM, Anthony Liguori wrote:
This may be acceptable, wait until the entire migration cluster is xzbrle capable before enabling it. If not, add a monitor command.
1) xzbrle needs to be disabled by default. That way management tools don't unknowingly enable it by not passing -no-xzbrle.
We could hook it to -M, though it's a bit gross.
That would needlessly prevent its use for any existing installed guests with a older machine type, which are running in a new QEMU Some kind of monitor capabilities seems good to me. Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
On 08/11/2011 12:16 PM, Daniel P. Berrange wrote:
On Thu, Aug 11, 2011 at 11:17:09AM +0300, Avi Kivity wrote:
On 08/10/2011 10:27 PM, Anthony Liguori wrote:
This may be acceptable, wait until the entire migration cluster is xzbrle capable before enabling it. If not, add a monitor command.
1) xzbrle needs to be disabled by default. That way management tools don't unknowingly enable it by not passing -no-xzbrle.
We could hook it to -M, though it's a bit gross.
That would needlessly prevent its use for any existing installed guests with a older machine type, which are running in a new QEMU
You could still enable it explicitly; I'm just trying to get it to be enabled by default.
Some kind of monitor capabilities seems good to me.
Live migration is probably mostly done in managed environments, so I think you're right. -- error compiling committee.c: too many arguments to function
On 08/11/2011 03:17 AM, Avi Kivity wrote:
3) a management tool should be able to query the source and destination, and then enable xzbrle if both sides support it.
You can argue that (3) could be static. A command could be added to toggle it dynamically through the monitor.
But no matter what, someone has to touch libvirt and any other tool that works with QEMU to make this thing work. But this is a general problem. Any optional change to the migration protocol has exactly the same characteristics whether it's XZBRLE, XZBRLE v2 (if there is a v2), ASN.1, or any other form of compression that rolls around.
If we have two-way communication we can do this transparently in the protocol itself.
Yes. This should be one of the initial caps to introduce.
Instead of teaching management tools how to deal with all of these things, let's just fix this problem once. It just takes:
a) A query-migration-caps command that returns a dict with two lists of strings. Something like:
{ 'execute': 'query-migration-caps' } { 'return' : { 'capabilities': [ 'xbzrle' ], 'current': [] } }
b) A set-migration-caps command that takes a list of strings. It simply takes the intersection of the capabilities set with the argument and sets the current set to the result. Something like:
{ 'execute': 'set-migration-caps', 'arguments': { 'set': [ 'xbzrle' ] }} { 'return' : {} }
c) An internal interface to register a capability and an internal interface to check if a capability is currently enabled. The xzbrle code just needs to disable itself if the capability isn't set.
Then we teach libvirt (and other tools) to query the caps list on the source, set the destination, query the current set on the destination, and then set that set on the source.
This is only if the capability has no side effect.
Right, it can't change the output of any monitor commands or anything like that. It's strictly about the encoding of the wire protocol which ought to be transparent to libvirt.
As we introduce new things, like the next great compression protocol, or ASN.1, we don't need to touch libvirt again. libvirt can still know about the caps and selectively override QEMU if it's so inclined but it prevents us from reinventing the same mechanisms over and over again.
Right.
Yes. But that negotiation needs to become part of the "protocol" for migration. In the absence of that negotiation, we need to use the wire protocol we use today. We cannot have ad-hoc feature negotiation for every change we make to the wire protocol.
Okay, as long as we have someone willing to implement it.
Sounds like a good hackathon project :-) Regards, Anthony Liguori
On Wed, Aug 10, 2011 at 02:27:41PM -0500, Anthony Liguori wrote:
On 08/10/2011 11:40 AM, Avi Kivity wrote: Instead of teaching management tools how to deal with all of these things, let's just fix this problem once. It just takes:
a) A query-migration-caps command that returns a dict with two lists of strings. Something like:
{ 'execute': 'query-migration-caps' } { 'return' : { 'capabilities': [ 'xbzrle' ], 'current': [] } }
b) A set-migration-caps command that takes a list of strings. It simply takes the intersection of the capabilities set with the argument and sets the current set to the result. Something like:
{ 'execute': 'set-migration-caps', 'arguments': { 'set': [ 'xbzrle' ] }} { 'return' : {} }
We have a number of commands to set migration parameters (bandwidth, max downtime, etc). One thing that has always troubled me with this, is that they are a global setting for QEMU, not simply the next migration command. This is fine if you expect that only a single 'migrate' command will ever be invoked during the lifetime of QEMU, but this doesn't hold true when we use 'migrate' as a means to implement saving of snapshots/save-to-file, or "core" dumps. For example, when libvirt does 'save to file', we want to set the max bandwidth to unlimited for that, but we don't want that 'unlimited' setting to apply to a future cross-host migration attempt. This means we have to send three commands migrate_set_speed 9223372036854775808 migrate file:/some/file migrate_set_speed 33554432 it doubly sucks because there is no way to reset the migration speed to the QEMU default, so we have to hardcoded (32 << 20) which is what QEMU currently uses. It would be more desirable if we could simply pass in the desired speed, compression algorithm, max downtime, etc, as parameters to the 'migrate' command. And then have 'migrate_set_speed' only affect the current active migration, not any future ones. So a 'query-migration-caps' command is nice, but I think having a set-migration-caps command is wrong. There should just be a 'caps' parameter for 'migrate'. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
participants (4)
-
Anthony Liguori -
Avi Kivity -
Daniel P. Berrange -
Shribman, Aidan