For those of you running EXT4 with Direct I/O on the likes of Intel Optane DC Persistent Memory or PMEM simulated via a virtual machine, better write performance is coming when overwriting already allocated blocks.

Queued within the EXT4 development code ahead of Linux 5.6 is optimize EXT4 DIO overwrites by SUSE's Jan Kara. This optimization about Direct I/O when overwriting already allocated blocks is particularly beneficial for multi-threaded workloads engaging in small writes.

When testing with FIO for performing 4K random writes, Kara found that the performance went up from 3018MB/s to 4059MB/s or when using direct access (DAX) the improvement went from 3042MB/s to 4311MB/s. This optimization just affects the performance when writing over already allocated blocks and won't have any performance improvements for reads or other non-DIO areas.

Quite the win for EXT4 Direct I/O performance come Linux 5.6!