Live Data Center Migration across WANs:A Robust Cooperative Context Aware Approach

Kobus Van der MerwewithK.K. Ramakrishnan and Prashant Shenoy

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Motivation

• Most network based services/applications involve components hosted in data centers

• Internet:– Mail/Web servers, VoIP, IPTV, P2P directory services etc

• VPNs:

– Mail servers, financial/business applications etc

• Many of these services require 24x7 availability• Any downtime is unacceptable

– At best inconvenience users; at worst major business impact; typically has financial implications

• Recent well published outages: Blackberry, Skype

• Objective our work:• Business continuity in face of data center outages, both

planned (planned maintenance) or unplanned (disaster recovery)

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Motivation cont.

Existing solutions to deal with outages are inadequate:• Local redundancy solutions

– Component redundancy (hot-swappable), multiple network connections • No protection against data center outages

• Existing cross data center solutions– Instance replication

• Same content/service available in multiple locations• Works well for stateless services (e.g., Web servers)

– Not for any statefull applications – Remote replication (either synchronous or asynchronous)

• Partial solutions– Typically only deals with storage– Not seamless; involves server downtime, IP addresses

change etc

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Our approach• Basic approach:

– Seamless live service migration across WANs• Including all components: server, data, network

• Cooperative, migration aware approach– Migration manager orchestrates migration across all three

subsystems• In summary:

– Planned outages• Migration of both server and data

– Live server migration– Performed once

• Atomic switchover of network to complete migration – Unplanned outages

• “Continuous live migration”– Server and data continuously replicated to remote site– On failure, atomic switchover of network

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Challenges

Seamless live server migration across WAN• LAN based live server migration enabled by virtual server

technologies (Xen, Vmware)

• WAN based server migration – Use existing virtual server migration

• “Management” connectivity to remote site to enable image migration– Network support to allow IP address to migrate with the (virtual)

server – Migrate storage to remote site

• Server and storage remain consistent

• Continuous live migration

LAN based live server migration:–The image of running virtual server is copied to a new physical platform (while the server is still running on the old platform)–Server state is synchronized between the two images–Migration software switches over to the new server with minimal downtime (tens of milliseconds) –New server is exactly the same as the old server (same IP address, network state stays intact etc)–Storage handled by through network attached storage (NAS), e.g., NFS

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Networking Support

• IP address migration:– Challenging to move IP addresses in current Internet

• Especially dynamically • Isolate impact on the rest of the network

– Routing protocols don’t change instantly– Connectivity changes not under data center control

• Our approach: – Allow migration management system to initiate network

connectivity change• Network provides API to migration manager

– Time critical changes are kept local• Network-wide (routing protocol) changes not time critical

– Use temporary tunnels to deal with mobility

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Physical Server (PS)

Virtual Server (VS)

PEa PEb

PSa

VSaPSb

VSa

PEc PEd

MigrationSoftware

Data CenterA Data Center

B

IP Migration Primitive

Network part of migration

1. Migration software signals to “network” that IPa will (soon) migrate from PEa to PEb

Goal: Migrate Virtual Server “a” (VSa) with IP address IPa from Physical Server “a” (PSa) in data center “A” (DCa) to Physical Server “b” (PSb) in data center “B” (DCb)

2. “Network” creates a tunnel between PEa and PEb

3. Server migration executed between PSa and PSb

4. Migration software signals to “network” that switchover should take place

5. PEa switches all traffic towards IPa to tunnel between PEa and PEb which delivers the traffic to VSa in PSb. (Return traffic does not need to go through tunnel.)

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IP Migration Primitive

PEa PEb

Data CenterA

Data CenterB

Physical Server (PS)

Virtual Server (VS)

PSa

VSaPSb

VSa

PEc PEd

After first five steps, server migration is done as far as migration software is concerned. Traffic towards IPa is “dog-legged” through PEa, so a few more steps remain in the network:

1. PEb starts to advertise a route to IPa with high local preference. So at this point there are two valid paths towards IPa, one though PEa and the tunnel and another directly through PEb. As routers start to learn about the newly advertised path they will prefer the direct path towards IPa and the tunnel will “dry out”.

2. When PEa detects no more traffic flowing through the tunnel it withdraws the route for IPa (if it had a specific route for IPa) and tears down the tunnel.

IP Migration Primitive:

Takes care of planned maintenance without storage needs

(E.g., VoIP network element)

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Data Storage

• Existing WAN solutions: remote replication – Maintain a primary/local and remote storage system– Replicate data between primary and remote systems– One of two modes:

• Synchronous: each write performed locally and remotely before return to “application”– Local and remote remains synchronized– Poor performance: both throughput and application latency

• Asynchronous: local and remote allowed to diverge, replicate a consistent “snapshot”– Good performance (high throughput, low (local) latency– Potential data loss because of divergence

Local RemoteSynchronous

Local RemoteAsynchronous

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Migration Aware Replication

Local Remote

Switch

Asy

nch

ronous

Synch

ronous

• Our approach:– Remote replication that can

seamlessly move between synchronous and asynchronous replication

– Allow replication mode to be controlled by migration management system: • Allow bulk of data to be replicated

asynchronously• Switch to synchronous when

needed– Final part of server migration

processIP Migration Primitive + Migration Aware Replication:

Takes care of planned maintenance with storage needs

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Unplanned Outages• Conflicting metrics of concern

– Recovery point objective (RPO)• How much data loss is acceptable?

– Recovery time objective (RTO)• How long can service be down?

– Cost (overhead of protection)• Range of meaning to “unplanned”

– Catastrophic instantaneous failure• No notice whatsoever

– But also imminent failure scenarios• Imminent equipments failure (e.g., increase in disk errors; imminent failure of fiber)• Developing natural/man-made disasters

– E.g., flooding/steam pipe burst in NY, probably even with 911– Minutes to hours to react

• Existing remote replication solutions deal with storage– No support for server migration

• Our goal:– Replicate data and server to allow for seamless failover

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Application state requirements•Limited application state:

– E.g., VoIP network element that maintains call state (for 3-way calling and mid-call events), or VoD servers (for fast-forward, random access events)

– Lost session state => application impact• Inconvenience

– RTO small, RPO medium • Some state loss is tolerable (drop few calls), but service has to stay up

– Instrument application to initiate partial migration, when new state has been created

• Statefull applications:– E.g., e-commerce applications (shopping cart, auction sites)– Lost session state => application impact

• (At best) inconvenience, (at worst)application correctness, monetary impact– RTO small, RPO small (minimize state loss, site has to stay up) – Continuous (incremental) server migration

• High integrity applications– E.g., financial transactions, other data base applications– RTO medium, RPO very small (absolutely no data loss, rather some

downtime)– Reduce RTO with continuous (incremental) server migration

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Continuous Server MigrationEnabling Technology: VS record/replay

Start Recording Snapshot of VS

Record execution state

Replay execution stateRestore snapshot

VSPS

REC

OR

DR

EPLA

Y

• Virtual server record/replay: available from VMware– Efficient recording: track “external” events + times– Synchronize events with VM state during replay– Developed as a debugging tool

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Continuous Server Migration With migration aware replication

Remote: REPLAY

Local: RECORDREPLICATE

• Asynchronously replicate initial snapshot

• Replication of execution state– Asynchronous if application can tolerate some state loss and execution state

represent consistent checkpoint– Synchronous otherwise

IP Migration Primitive + Migration Aware Replication + Continuous Server Migration:

Takes care of unplanned outages

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Status

• Migration aware replication– Key building blocks prototyped

• “Semantic Aware Replication” project• Gal Niv (UMass)

• WAN live migration– Key building blocks prototyped (without storage)

• “Live virtual router migration” project• Yi Wang (Princeton)

• Continuous Server Migration– Just getting off the ground

• Work in progress– Many open issues remain!