virtual machine - Understanding availability set in Windows Azure -


i reading explanation of availability sets on microsoft' website can't 100% understand concept.

http://www.windowsazure.com/en-us/documentation/articles/manage-availability-virtual-machines/

there many questions people ask in comments, there no technical support microsoft there answer them.

as understand availability sets can duplicate vm iis application , vm sql, means have use 4 vm(pay 4) instead of 2. means whenever iis1 virtual machine down, website still online of iis2 virtual machine , vice versa? same goes sql1 , sql2 virtual machines?

am going right direction? if case, how keep data synchronized in sql1 , sql2, iis1 , iis2 virtual machines @ same time, website still latest data , code if 1 vm down updates?

an availability set combines 2 concepts windows azure paas world - upgrade domains , fault domains - make service more robust. when several vms deployed availability set windows azure fabric controller distribute them among several upgrade domains , fault domains.

a fault domain represents grouping of vms have single point of failure - convenient (although not precisely accurate) way think rack single top or rack router. deploying vms different fault domains fabric controller ensures single failure not take entire service offline.

the fabric controller uses upgrade domains control manner in host os upgrades (i.e., of underlying physical server) performed. fabric controller performs these upgrades 1 upgrade domain @ time, moving onto next upgrade domain when upgrade of preceding upgrade domain has completed. doing ensures service remains available, although reduced capacity, during host os upgrade. these upgrades appear happen every month or two, , services in vms deployed availability sets receive no warning since supposedly resilient towards upgrade. microsoft provide warning upgrades subscriptions containing vms deployed outside availability sets.

furthermore, there no sla services have vms deployed outside availability sets.

as regards sql server, may want use of sql server availability groups sit on top of windows server failover cluster , use synchronous replication of data. iis, may want @ possibility of deploying application paas cloud service since provides significant advantages on deploying iaas cloud service. can create service topology integrating paas , iaas cloud services through use of vnet.


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