UEFI 2.4 Review, Part 5: EFI_DISK_IO2_PROTOCOL to support asynchronous I/O

This is the fifth part of an on-going series of articles examining the changes in the UEFI 2.4 specification. This week we are examining a brand new protocol, the EFI_DISK_IO2_PROTOCOL. As the "2" indicates, this is an enhanced version of a protocol that existed previously. In this case, the EFI_DISK_IO_PROTOCOL has existed for a long time, at least back to the EFI days. These protocols both give byte oriented access to the contents of a physical or virtual device.

The EFI_DISK_IO_PROTOCOL was typically layered on top of the EFI_BLOCK_IO_PROTOCOL. The UEFI 2.3.1 specification added the EFI_BLOCK_IO2_PROTOCOL. This provided asynchronous access to storage devices at the block level, so that processing could continue while the blocks were being read or written.

But the same specification never updated the EFI_DISK_IO_PROTOCOL. Since EFI_DISK_IO_PROTOCOL was still synchronous (requiring all reads or writes to continue before returning) and all of the file system related protocols use it, they could not take advantage of the performance gains offered. This was because the ReadDisk() and WriteDisk() functions were defined to wait for all reads or writes to complete before returning. So even if they used EFI_BLOCK_IO2_PROTOCOL, they couldn't return any sooner.

Now, with the new EFI_DISK_IO2_PROTOCOL protocol and related updates to the EFI_FILE_PROTOCOL protocol, the file system drivers (such as those for FAT32 or El Torito or even EXT2/3/4) can offer enhanced performance by allowing processing to continue while disk operations complete.

We will look at the enhancements to the EFI_FILE_PROTOCOL in more detail in a later article.

UEFI 2.4 offers enhanced storage performance, which can greatly speed up boot time and other disk-bound activities.