Re: [PATCH] ext4: rralloc - (former rotalloc) improved round-robin allocation policy

[Mario Lohajner <mario_lohajner@rocketmail.com>]

On 03. 03. 2026. 02:33, Theodore Tso wrote:
> On Mon, Mar 02, 2026 at 09:04:44PM +0100, Mario Lohajner wrote:
>> RRALLOC spreads allocation starting points across block groups to avoid
>> repeated concentration under parallel load.
> 
> There are already other ways in which we spread allocations across
> block groups.  You need to tell explain a specific workload where this
> actually makes a difference.
> 
> Also note that in most use cases, files are written once, and read
> multiple times.  So spreading blocks across different block groups is
> can often be actively harmful.
> 
>> In high-concurrency testing, performance is consistently comparable to
>> or occasionally better than the regular allocator. No regressions have
>> been observed across tested configurations.
> 
> No regressions, and only "occasionally better" not enough of a justifiation.
> 
> What is your real life workload which is motivating your efforts?
> 
>       	     	       		      	 - Ted

RRALLOC targets sustained parallel overwrite-heavy workloads such as
scratch disks, rendering outputs, database storage and VM image storage.

It introduces a round-robin allocation policy across block groups to
reduce short-term allocation concentration under high concurrency.

It is not intended to improve write-once/read-many workloads and remains
disabled by default.

I do understand that without clearly measurable workload-specific 
improvement, this is likely not sufficient justification for upstream 
inclusion.

I will continue evaluating and refining the allocator out-of-tree.
If I am able to demonstrate concrete and reproducible benefits beyond
allocation geometry and occasional contention-related effects,
I will revisit the discussion with additional data.

Thank you for the review and valuable feedback.

Regards,
Mario Lohajner