The query role’s responsibility is to respond to search requests from the web role and secondly to maintain a propagated copy of the index stored on the Query servers local file system. It is a good candidate for virtualization, as long as SharePoint architects take a few key factors into consideration:
- For best performance, use fastest disk infrastructure possible: Prefer dedicated physical volumes on underlying SAN infrastructure using the “pass through” disk feature of Hyper V or fixed disk VHDs on that LUN over dynamically expanding virtual hard disks. Performance of a fixed size VHD is practically en par with pass-through disks and has management and backup advantages.
- Don’t put your query and index server on the same underlying physical disk: Index server is heavy read write while query server is constantly updating its own copy of the index. Therefore contention to underlying disk will slow read I/O for your query servers in your MOSS Farm.
- Combine or split the Web/Query role: SharePoint architects often combine web and query roles. In the virtualized environments this is also a good practice but it largely depends on your environment and performance requirements. Virtualization environments provide the flexibility to split combined web/query role onto separate machines.