Real Time Issues in Live migration of Virtual Machines

Presented by : Ran Koretzki

Basic IntroductionWhat are VM’s ?What is migration ?What is Live migration ?

What are VM’s? VM’s (Virtual Machines) - "completely isolated

guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or (in most cases) both together.

This allows to run multiple independent VM’s on a single physical Machine.

VM’s operating systems are not hardware depended.

What are VM’s?

Traditional Architecture Virtual Architecture

Benefits What are VM’s? Hardware independence.

Encapsulation - VMs can be described in a filePossible to ‘snapshot’.Easy to move and to backup.Easy to clone and scale wide a server application.Many VM venders : VMware, Microsoft , Citrix…

Enables running multiple operating systemsConsolidation & use of unused computation power.Resource management.High availability & disaster recovery.

Easy Management.

Migration – next on the agenda.

MigrationDefinition - The ability to move VM’s from one PH to another.

In the past, to move a VM between 2 PH, it was necessary to shutdown the VM, allocate the needed recourses to the new PH, move the VM files, and start the VM in the new host.

The recourses that need to be transferred are : memory, the internal state of the devices and of the virtual CPU. The must time consuming to transfer is memory.

The problem : downtime. The solution was at first Automation, but the real improvement came with Live Migration.

Live MigrationWiki Definition - allows a server administrator to move a

running virtual machine or application between different physical machines without disconnecting the client or application. For a successful live migration, the memory, storage, and network connectivity of the virtual machine needs to be migrated to the destination.

It mean it allows the server admin to move VM’s between PH transparently to the clients.

It is done usually for Load balance between PH and for migration in case of a hardware failure.

• Live migration of virtual machines

• Zero downtime

Real Time Issues in Live migration of Virtual Machines

Presented by : Ran Koretzki

By:Fabio Checconi, Tommaso Cucinotta, Manuel Stein.

Objectives of the paper1. Show a heuristic to reduce the

downtime of a VM during live migration by scheduling which memory pages to transmit first.

Assumptions, where is a memory page of size P.

Available bandwidth for the transfer - b (bps).

Time needed to transfer a single page - where H is the overhead

Each page will be accessed for “write” with a probability of during each time frame T.

Algorithm1. At time (start of Migration)- is a set of pages to be

transmitted. It is set to be the entire page set used by the VM.

2. For do: all the pages in are transferred according to the order specified by ;

The transfer ends at pages in are found dirty again;

3. Stop the VM and transfer the last pages, up to migration finishing time using a bandwidth of bps, with .

Crucial ValuesDown Time (meaning VM not available) is -

Overall Migration time :

Propositions1. The probability of a page that is not dirty at time (start of migration) to become dirty and thus need to be transmitted in the final migration round is:

2 .The probability of a page that is dirty at time (start of migration) to become dirty and thus need to be transmitted in the final migration round is:

(Where denotes the inverse of the function).

The expected overall migration time (with K = 1)

Order of TransmissionThe order of transmission of the pages that minimizes the expected number of dirty pages found at the end of the live migration step must satisfy the following condition:

Conclusion: If the probabilities are lower than the optimum ordering is obtained for increasing values of the probabilities On the other hand, if the probabilities are greater than the optimum ordering is obtained for decreasing values of the.

Reducing migration timeAll pages are equal, but some are more equal

Problem: wasteful to transmit at each stepSolution: Wait until the end, when VM is down

Algorithm: among the pages that are found as dirty at start of step k, for a set of pages delay the transmission to when the VM is stopped. Which pages? (where is a threshold value)

Conclusion : It is possible to achieve a negligible increase in down-time with a substantial decrease of overall migration time.

A problem with this modelProblem: Need to know, precisely, for each page .

Solution: Gathering information during run-time. (Statistics)

Problem: Non-negligible overheads.

Solution: LRU - Frequency-based approach

ResourcesComputational Resources:

Scheduling Guarantee by the Kernel – (Q, P).Meaning period P and share Q

Network Resources:b needs to be constant and Possible to reserve.An unstable network is not part of this model, but a migration will still succeed (but it may not be Live Migration)

Simulation of the algorithmThe authors have modified the KVM hypervisor.

Page Tracing mechanismPage accessed are traced within the hypervisor, using a bitmapThe implementation will exploit this information to modify transfer order

Simulations – Virtualized VideoLAN Client (VLC) as a streaming server

6500 mapped pages (16KB/page)Transfer rate of 100MBit/sec.8 sec. (!)

Simulation of the algorithm

Guaranteed bandwidth of 50 Mbit/sStandard vs. LRU

570 -> 300 (47%) (K=1)360 -> 290 (19.4%) (K=3)4800 -> 4500 (6.25%) (K=1)5500 -> 5000 (9.1%) (K=3)

Simulation of the algorithm

LRU with delayed transmissionLRU vs. Improved LRU

300 -> 220 (27%) (K=3)5000 ->4400 (12%) (K=3)

Conclusions• It’s possible to minimize downtime and improve

QoS with simple page ordering algorithms

• With a certain bandwidth, LRU has been proved as an effective aid for page ordering, achieving good results.

• Further Work needs to be done…