but the vmware document say that i should use dedicated storage for the esxi host to get better performance.
This is for better performance or best practices.
in contradiction to that, vmware letting install the hypervisor on slow sd card...
There is no contradiction here. If you have a USB or SD card, you will bypass the traffic from the vm's access to disks and the hypervisor's access to the disks.
how much i/o (if any) put strain on the esxi install disk?
Well, if that is your actual question, then there is no exact list to be given, but of the administrator's prespective to choose.
However, considering your question, if you take a look at the / partitions of the hypervisor, you mainly use it for booting, dumping the iso files for tools installations, dumping the logs [which could be increasing on having a superior infrastructure] and most importantly, storing the configuration files and the process nodes in runtime [vsish]
The amount of strain you put is entirely based on the result of swapping indirectly. For example, IF you have high config VMs in datastores which request more RWs, then the access of the configuration files, loggings might be impacted or slower. But if you try to avoid the swapping [writes to disks], then you can have vms and esxi happily living together in local disks itself.
Again, when you have a RAID failover and RAID rebuilt in process, it would be both the hypervisor as well as the VM read/writes to the disks getting slower.
Hope it helps,
zXi
