On 08/08/2011 10:15 AM, Avi Kivity wrote:
On 08/08/2011 06:10 PM, Anthony Liguori wrote:
On 08/08/2011 09:47 AM, Avi Kivity wrote:
On 08/08/2011 05:46 PM, Avi Kivity wrote:
Please provide documentation in docs/ of the compression format.
IMO it should be disabled by default (with an option to disable it,
via, sat, migrate-set-options, so we can migrate to older hosts).
The protocol should allow XBZRLE to turn itself off if it detects that
it isn't effective.
IOW, this should be part of the standard migration protocol, not some
side option that is enabled if the user remembers. It should not be
mutually exclusive with future migration extensions, including
compression.
Are you thinking of a static decision or a dynamic decision?
Dynamic. If the cache hit rate is too low, disable XBZRLE and eliminate
the overhead of copying pages to the history buffer.
I think feature negotiation would address static decision making. For
dynamic decision making, you could look to something like the VNC
protocol and how it encodes pixel data. The flow looks something like:
1) All clients/servers must support raw encoding
2) Client presents list of support encodings
3) Server takes intersection of client supported encodings and server
supported encodings.
4) Server can choose to encode updates using any encoding supported by
client and server.
Feature negotiation in the migration protocol itself would break exec:
migration (and any existing single duplex proxies).
We can do a poor man's feature negotiation via capabilities, relying on
management to disable features which don't exist on the other side. It
isn't pretty, but it's the best we can do at this point.
I think the above can be done via capabilities too fwiw. In this case,
the source and destination advertise the compression formats they
support, and the management tool takes the interaction and sets the
capability mask on the source and destination appropriately.
There's no need for a full duplex protocol because the source just sends
compressed data in whatever format it thinks is appropriate at any given
point in time.
Real feature negotiation will likely have to wait until the next version
of the migration protocol.
Since we're talking about moving to ASN.1 for 1.0, I think we should
think also include memory compression and wait until we rev the protocol
before introducing any type of compression.
Regards,
Anthony Liguori
