What is VM Migration in Cloud computing?
By: Sarthak Goyal and Vishal Gupta
What is
Virtual Machine migration?
Virtual Machine
migration is a technique of shifting a working virtual machine between different
other physical machines while having a connection with the client and application.
It means that we should not disconnect the client. Memory, storage, Network
connectivity of a virtual machine is transferred from the original guest
machine to destination.
Why is it needed?
a)
Load
balancing:
Move VMs to a less busy host.
Make use of newly added capacity.
b)
Recovery
from host failure
Types of VM Migration:
a ) Cold Migration b) Live
Migration
a) Cold
Migration:
Cold Migration is the migration of
powered off virtual machines between hosts across clusters and data centers.
With Cold migrations, the use of data stores is more flexible. Cold Migration for virtual machines may allow more effective transfer with less complexity.
Figure 1. Cold Migration
A Cold
Migration consists of the following operations:
1)
If
we are shifting it to another datastore, then the configured files, like
files which are suspended or the log files are shifted from the source host to
the storage area of the destination host. The virtual machine disks can also be
shifted.
2)
The virtual machine is attached to the new host.
3)
When
the process of migration is done, the existing model of a virtual machine is
removed from the source host. It will also be deleted from datastore if you
will choose that option.
b) Live Migration:
Live migration is
a technique of shifting a working virtual machine between different other physical
machines without disconnecting the client or application. While doing so the Memory,
storage and network connectivity of the virtual machine are transferred from
the original guest machine to the destination.
Figure 2. Live Migration
Live Migration is
a technology used for load balancing and optimization of VM deployment in data
centers. By using the process of live migration, a virtual machine can be transferred
to another node without switching off the machine. Live migration is classified
into two steps:
·
Control
is switched to the destination
·
Data
transferring to the disk
Two techniques for
moving the VM’s memory state from source to destination are Pre-copy memory
migration and post-copy migration.
Pre-copy
memory migration:
Warm-up phase- In this
phase all the pages are copied from source to destination host while virtual the machine is still running on the host. If some memory pages change during memory
copy process (dirty pages), they will be re-copied again until the rate of
recopied pages are not less than the dirty pages rate.
Stop and copy phase- At this phase, the virtual machine will be stopped from working on the
source host so that the dirty pages which are left will be copied to the
destination and then the virtual machine will be resumed on the destination.
Post-copy
memory migration:
It
defers from pre-copy memory migration in the sense that it transfers the VM’s
memory contents until after its processor state has been sent to the target
host while in traditional pre-copy approach firstly memory state is copied over
multiple iterations and then processor state is transferred finally.
Figure
3. Pre-Copy Algorithm
Figure 4. Timeline for Post-copy and
Pre-copy
Future scope:
VM Migration SLA based- There is a limitation of VM Migrating technology, when
it is used for SLA (Service Level Agreement) the optimized data access might
still, go beyond the time requirement.
High Dirty Rate Memory Page- The problem like more page fault and higher total
migration time occurs when there is a high dirty rate of memory pages and in this
case, even the memory threshold technique is applied.
References:
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