An I/O request – from start to finish

Now that we have a general understanding of how ESXi presents storage to a virtual machine and handles load balancing and failover, let's have a look at an I/O request from start to finish. The following figure shows a graphical representation of the following steps:

• The VM issues a SCSI request to its respective virtual disk.

• Drivers from within the guest OS communicate with the virtual storage adapters.

• The virtual storage adapter forwards the command to the VMkernel where the PSA takes over.

  • The PSA loads the specific MPP (in our case the NMP) depending on the logical device holding the virtual machines disk.

  • The NMP calls the associated PSP for the logical device

  • The PSP selects the appropriate path to send the I/O down while taking into consideration any load balancing techniques. The I/O is then queued to the hardware/software initiator, CNA, or HBA depending on the storage transport being used.

  • If the previous step fails, the NMP calls the appropriate SATP to process error codes and mark paths inactive or failed, and then the previous step is repeated.

• The hardware/software initiator, CNA, or FC HBA transforms the I/O request into the proper form depending on the storage transport (iSCSI, FC, or FCoE) and sends the request as per the PSAs instructions.

  

  I/O flow from start to finish