Search index - and high availability

Question

Running Search on WFE(A) and APP(B) server - with this topology.
The company relies heavily on the Search Index - so a reset of the index isn't always an option, and the re-build takes a lot of time, so in order to avoid a index reset. 
Is it possible to build a server X that holds the Index copy for server A+B and then when a reset is necessary on server A+B - tell SP2013, it now have to utilize the index that "sits" on server X, while rebuilding is done on server A
+ B?
Can I build a separate index for a specific content source/site collection? - so the content source
e.g. "http://contoso.com" are in search indexY and the "http://contoso.com/sites/" is in IndexZ   - and that way only reset indexZ and not touch indexY? 
Does a second "Search Service Application" contribute/solve any? 

Answer 1

Hi,
1.Yes,you can add a new index partition consists of server A and server X(replica) , and add another index partition consists of server B and server Y(replica). For more information, you can refer to the article:
http://technet.microsoft.com/en-us/library/jj862355(v=office.15).aspx#Search_Index_Part
2.No，you cannot. But you can reset index at site level and list level.
https://support.office.com/en-US/article/Manually-request-crawling-and-reindexing-of-a-site-9afa977d-39de-4321-b4ca-8c7c7e6d264e
3.No, you donot need to create a seacond "Search Service Application".
Thanks,
Eric
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