Hi,

I finally have an update for the proposal ready, after some thinking and throwing some ideas around with colleagues we came to the conclusion that the best approach might be as followed (without having dynamic return values)


I have set up some test engine with 'FakeVDSM' and I am tracking the data sent. The system has 250 hosts and about 10000 VMs running. I have captured 1.3 GiB TCP data within about 20 minutes only from the communication between the engine and the fake VDSM host. In a frame of 23 minutes I have captured 13.560 getAllVMStats and getAllVdsStats calls, and 67827 calls to list.

getAllVmStats replies have produced in that time about 510MiB of data transmitted to the engine. This proposal below would reduce the overall amount of data significantly and we're preparing to work on a prototype for the engine backend as well so we can backup this statement with hard numbers.


Here's the proposal.


   VDSM <=> Engine data retrieval optimization


     Motivation:

Currently the oVirt Engine is polling a lot of data from VDSM every 15 seconds. This should be optimized and the amount of data requested should be more specific.

For each VM the data currently contains much more information than actually needed which blows up the size of the XML content quite big. We could optimize this by splitting the reply on the getVmStats based on the needs of the engine into different requests. For this reason Omer Frenkel and me have split up the data into parts based on their usage.


     Changes


       New Verbs


         getAllRuntimeStats

Get runtime information of all VMs
Returns for each VM a map with UUID and a value of:

 * *@cpuSys* Ratio of CPU time spent by qemu on other than guest time
 * *@cpuUser* Ratio of CPU time spent by the guest VM
 * *@memUsage* The percent of memory in use by the guest
 * *@elapsedTime* The number of seconds that the VM has been running
 * *@status* The current status of the given VM
 * *@statsAge* The age of these statistics in seconds
 * *@hashes* Hashes of several statistics and information around VMs

Hashes consists of:

 * *@info* Hash for VmConfInfo data
 * *@config* Hash of the VM configuration XML
 * *@status* Hash of the VmStatusInfo data
 * *@guestDetails* Hash of the VmGuestDetails data


         getStatus

Get status information about a list of VMs
Parameters:

 * *@vmIDs* a list of UUIDs for VMs to query

Returns for each VM in vmIDs a map with UUID and a value of:

 * *timeOffset* The time difference from host to the VM in seconds
 * *monitorResponse* Indicates if the qemu monitor is responsive
 * *clientIp* The IP address of the client connected to the display
 * *username* the username associated with the current session
 * *session* The current state of user interaction with the VM
 * *guestIPs* A space separated string of assigned IPv4 addresses
 * *pauseCode* Indicates the reason a VM has been paused


         getConfInfo

Get configuration information about a list of VMs
Parameters:

 * *@vmIDs* a list of UUIDs for VMs to query

Returns for each VM in vmIDs a map with UUID and a value of:

 * *acpiEnable* Indicates if ACPI is enabled inside the VM
 * *displayPort* The port in use for unencrypted display data
 * *displaySecurePort* The port in use for encrypted display data
 * *displayType* The type of display in use
 * *displayIp* The IP address to use for accessing the VM display
 * *pid* The process ID of the underlying qemu process
 * *vmType* The type of VM
 * *kvmEnable* Indicates if KVM hardware acceleration is enabled
 * *cdrom* /*optional*/ The path to an ISO image used in the VM's
   CD-ROM device
 * *boot* /*optional*/ An alias for the type of device used to boot the VM


         getAllDeviceStats

VM device statistics containing information for getting statistics and SLA information
Returns for each VM a map with UUID and a value of:

 * *memoryStats* Memory statistics as reported by the guest agent
 * *balloonInfo* Guest memory balloon information
 * *disksUsage* Info about mounted filesystems as reported by the agent
 * *network* Network bandwidth/utilization statistics
 * *disks* Disk bandwidth/utilization statistics


         getGuestDetails

Get details from the guest OS from a list of VMs
Parameters:

 * *@vmIDs* a list of UUIDs for VMs to query

Returns for each VM in vmIDs a map with UUID and a value of:

 * *appsList* A list of installed applications with their versions
 * *netIfaces* Network device address info as reported by the agent


       Usage

Currently the engine is requesting currently every 3 seconds the vm list from each vdsm host and every 15 seconds all the data mentioned above for all VMs.

The change would be as follows:

The engine requests every 3 seconds getAllRuntimeStats from vdsm which will give the engine the most used data. If the engine has a mismatch of the hashes returned by getAllRuntimeStats it should request the data changed.

if hashes.info changed => request getConfInfo with all vmIDs on that host where the hash changed if hashes.status changed => request getStatus with all vmIDs on that host where the hash changed if hashes.guestDetails changed => request getGuestDetails with all vmIDs on that host where the hash changed

Request getAllDeviceStats periodically e.g. every 5 minutes, which should be sufficient for the DWH, in case it is not it could be even configurable.



On 03/17/2013 04:30 PM, Dan Kenigsberg wrote:
On Sun, Mar 17, 2013 at 10:28:15AM -0400, Ayal Baron wrote:

----- Original Message -----
On 17/03/13 15:13, Ayal Baron wrote:
----- Original Message -----
On 03/13/2013 11:55 PM, Ayal Baron wrote:
...
The only reason we have this problem is because there is this
thing against making multiple calls.

Just split it up.
getVmRuntimeStats() - transient things like mem and cpu%
getVmInformation() - (semi)static things like disk\networking
layout
etc.
Each updated at different intervals.
+1 on splitting the data up into 2 separate API calls.
You could potentially add a checksum (md5, or any other way) of
the
"static" data to getVmRuntimeStats and not bother even with
polling
the VmInformation if this hasn't changed.  Then you could poll
as
often as you'd like the stats and immediately see if you also
need
to retrieve VmInfo or not (you rarely would).
+1 To Ayal's suggestion
except that instead of the engine hashing the data VDSM sends
the
key which is opaque to the engine.
This can be a local timestap or a generation number.
Of course vdsm does the hash, otherwise you'd need to pass all
the
data to engine which would beat the purpose.
I thought you meant engine will be sending the hash of previous
requests
per VM to vdsm, then vdsm will reply back with vm's removed, vm's
added,
and the details for vm's that changed (i.e., engine would be doing
something like if-modified-since-checksum per vm).
benefit is reducing a round trip.
but first would need to split to calls of stats (always changing)
and
slowly/never changing data.
If vdms accepts the hash then in your method engine would have to
periodically call getVmInfo(hash).
What I was suggesting is that getVmStats would return vmInfo hash
so that we could avoid calling getVmInfo altogether.
The stats *always* change so there is no need for checking if that
info has changed.
What we could do is avoid the split into 2 verbs by calling
getVmStats(hash) and then have getVmStats return everything if the
hash has changed or only the stats if it hasn't.  This would be
the least number of roundtrips and avoid the split.  If you don't
pass a hash it would return everything so this way it's also fully
backward compatible.
For the 'static' data, why is there a need for a hash?
If VDSM sends in each update a timestamp, can't RHEVM just use
if-modified-since with the last timestamp it got from VDSM?
Is it cheaper for VDSM to calculate the hash, than update the
timestamp
per change in any of the fields? It doesn't really need to update the
timestamp per change, only for the first change since last update
sent
actually (so 'dirty' flag in a way, to signify data that RHEVM hasn't
seen yet).
Y.
As Saggi mentioned: "VDSM sends the key which is opaque to the engine. This can be a 
local timestap or a generation number."

The content doesn't matter, what matters is that it has changed.
timestamp assumes that vdsm will track changes and send only delta.
Although possible this would be an overkill (for every value in the
dict you'd have to hold a timestamp of last change and send only those
which have changed since the timestamp which was passed by the user).
If we're in the spirit of quoting Saggi, this suggestion is not
compatible with "...mak[ing] the return value differ according to input
... is a big no no when talking about type safe APIs.".

Dan.
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--
Regards,

Vinzenz Feenstra | Senior Software Engineer
RedHat Engineering Virtualization R & D
Phone: +420 532 294 625
IRC: vfeenstr or evilissimo

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