Re: [RFC] AceDelta: OTA delta for EROFS images, 30-40% smaller than AOSP 17 delta_generator

[Gao Xiang <xiang@kernel.org>]

Hi,

(+cc Kelvin on this topic)

On Wed, Oct 07, 2026 at 10:58:10PM +0800, peacherwu wrote:
> Hi,
> 
> I'd like to share results of an alternative OTA delta generator/applier
> for EROFS partition images.
> 
> AceDelta diffs whole EROFS partitions (system, vendor, odm, my_* etc.) at
> file level, recompressing lz4 clusters on apply. Compared with AOSP 17
> delta_generator (minor_version 10, -enable_lz4diff)
> on seven real consecutive-build pairs from five phones:
> 
>   Device / builds                    AOSP MB  AceDelta MB  Reduction
>   OnePlus 15      A.22 -> A.27        1152.6      683.7      40.7%
>   OPPO Find X8 Pro C.76 -> C.79       1059.1      733.1      30.8%
>   Xiaomi 15 Pro   2.0.105 -> 2.0.214  1038.6      655.6      36.9%
>   Xiaomi 15 Pro   3.0.3 -> 3.0.7       833.4      490.4      41.2%
>   Xiaomi 15 Pro   2.0.214 -> 3.0.3    2657.9     1685.8      36.6%
>   Nubia Z80 Ultra 16.0.12 -> 16.0.16   767.9      514.7      33.0%
>   Realme GT8 Pro  A.68 -> A.71        1268.9      878.1      30.8%
> 
> Generation is ~2.5x faster than delta_generator; apply time (on PC) is
> about the same (single-thread 8% slower, multi-thread 20% faster using more
> RAM). Hash tree and FEC are
> regenerated at apply time by my own implementations. Detailed info, plus
> Linux evaluation binaries:
> 
>   https://github.com/peacherwu969/acedelta

That is a great effort. If you're interested, you could seek
Google Android staffs and see if they are interested.

> 
> A question for EROFS people. Both AOSP (lz4diff) and AceDelta must
> reproduce the lz4 compression exactly; mismatches are patched with small
> fixups. Although AceDelta is doing better than AOSP in fixup sizes through
> lots of experiments, it's still like shooting in the dark. Is there a more
> systematic way that ensures exactness?

I'm not sure if it has a better approach since it relies on a strict
compressed data reproducibility of the compressor.)

> 
> Is there an EROFS WeChat group I could join?

I will message you my own WeChat number privately since I tend to
avoid leaking my contact info publicly.

Thanks,
Gao Xiang

> 
> Thanks,
> --Peacher