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What is the best setting for virtual machine to use the Guest mode random accessing files on disk (many IOPS) and also to use the same virtual machine as a file server to access the same files many IOPS? Detail: I do not need very high transfer rate of serial data on the disk, but a high rate of read-write random access and high IOPS rates in the network as well.

Thank you!

I find your question somewhat contradictory: there are no guest settings that improve host disk performance, and there are no settings on host or guest that will truly improve random access speeds. The best you can hope for is that host caching will reduce the cost of non-random (i.e. repeated) accesses. I also doubt that hdd performance is very significant next to network latencies.

That all said: you would use true network shared folders (not GA shares), you would use VirtIO-net drivers, and you would ensure that host and guest disks are defragmented.