How VMware vMotion works ?
VMware vMotion enables live migration of running virtual
machines from one physical to another with zero downtime, continuous service availability
& complete transaction integrity. This feature improves availability of conducting
maintenance without disrupting business.
VMware vMotion is enabled by three underlying technologies:
1) Entire state of virtual machine is encapsulated
by set of files stored on shared storage
2) Active memory page & system state of virtual
machine (preCopy) is rapidly transferred over high-speed vMotion network
allowing to switch from source host to destination host. Keeps track of
on-going memory transaction in a memory bitmap. Once entire memory & system
state are copied to destination host, the source virtual machine is Quiesced.
Memory bitmap does not have contents of memory; instead it has addresses of
that memory (also called dirty memory). Target host reads the addresses in the
memory bitmap file and requests the contents of the addresses from the source
host. After copying the bitmap to target host, the virtual machine resumes on
the target ESX host. The entire process takes < 2 seconds.
3) The network is also virtualized, ensuring even
after the migration virtual machine network identity & network connections
are preserved. VMware vMotion manages Virtual MAC. Once the destination machine
is activated, vMotion sends RARP message to the physical switch to ensure that
it is aware of the new physical location of the virtual MAC address. After virtual
machine successfully operating on the target host, memory on the source host is
deleted.
VMware vMotion also migrates resource allocation (CPU &
memory) from one host to another. On a bad day, ping -t may result in loss of
one ping packet. Applications can withstand loss of more than a packet or two.
VMware vMotion requirements:
1) Shared storage
2) Gigabit Ethernet NIC with VMkernel port defined
for vMotion on each ESXi hosts
A Successful vMotion relies on the following host conditions:
1) Source & destination host must be configured
with identical virtual switches with vMotion enabled port group.
2) All port groups to which virtual machine being
migrated must exists on both ESXi - case sensitive, VLANs.
3) Processors must be compatible.
A successful vMotion relies on the following virtual machine conditions:
1) Virtual machine must not be connected to any
physical device available only to one ESXi host.
2) Virtual machine must not be connected to an
internal-only virtual switch.
3) Virtual machine must not have CPU affinity set
to specific CPU.
4) Virtual machine must have all the files on VMFS
or NFS datastore accessible to both ESXi hosts.
High priority migration does not proceed if the resources
aren’t available to be reserved for the migration. Standard priority migration might
proceed slowly and might fail to complete if enough resources aren't available.
At 14%, a pause occurs while the hosts establish communications
and gather information for pages in memory to be migrated.
And at 65%, another pause occurs when the source virtual machine is quiesced and dirty
memory pages are fetched from the source host.
Sometimes, the vMotion progress action fails at certain percentage.
Below are the reasons for the vMotion failure at certain percentage:
1)
9% - issue with the ESXi NICs – upgrade the NIC
firmware to resolve the issue
2)
10% - datastore of the VM mounted was mounted
read-only – needed read/write access
3)
10% - log.rotateSize
value in the virtual machine's .vmx file is set to a very low value, it causes
the vmware.log file to rotate so
quickly that by the time the destination host is requesting the vmware.log
file's VMFS lock. The destination host is then unable to acquire a proper file
lock, and this causes the vMotion migration failure.
4)
14% - fails if there are multiple VMkernel ports
in the same network (or) incorrect VMkernal interfaces selected for vMotion.
5)
78% - NFS storage path UUID mismatch
6)
82% - caused by incorrect datastore information
or out-of-date virtual hardware.
7)
90% - attempting to vMotion 64-bit virtual
machines from an ESX/ESXi host with VT enabled in the BIOS to an ESX/ESXi host
with VT not enabled in the BIOS.
8)
90% - occurs if the host Migrate.MemChksum value is set to 1, but the other hosts are set to
0.
