[U-Boot] EXT4 slow boot problem

[Pendleton, Blair]

We have discovered a significant performance problem in U-Boot accessing an EXT4 file system.    If a large image file happens to be created with more than 4 extents, file read time increases from a few seconds to several minutes.     The root cause is the indexed extent block is being re-read tens of thousands of times from the underlying device.

In our case, the image file is ~44MB and the underlying device is a MMC SD card.    Software updates are performed in Linux, and depending on a particular instance of EXT4 block allocations, the image file is created with 3, 4 or 5 extents.    With 3 or 4 extents (tree depth 0), file reads are fast because the extent records are located in the cached i-node and the entire file is read with 3 or 4 large MMC block reads.     With 5 extents (tree depth 1), extent records are in a separate block referenced by an extent index node.   The current U-Boot implementation makes no attempt to cache this block and is forced to re-read it for every block of file data (EXT4 block size 1KB).   This results in excess of 43000 MMC read operations and boot times so long that the system appears to be hung.

Blair Pendleton
Fluke Networks R&D
Colorado Springs, CO, USA