Re: [RFC PATCH 03/24] erofs: add Errno in Rust

[Ariel Miculas]

On 24/09/26 04:25, Gao Xiang wrote:
> 
> 
> On 2024/9/26 16:10, Ariel Miculas wrote:
> > On 24/09/26 09:04, Gao Xiang wrote:
> > > 
> 
> 
> ...
> 
> > 
> > And here [4] you can see the space savings achieved by PuzzleFS. In
> > short, if you take 10 versions of Ubuntu Jammy from dockerhub, they take
> > up 282 MB. Convert them to PuzzleFS and they only take up 130 MB (this
> > is before applying any compression, the space savings are only due to
> > the chunking algorithm). If we enable compression (PuzzleFS uses Zstd
> > seekable compression), which is a fairer comparison (considering that
> > the OCI image uses gzip compression), then we get down to 53 MB for
> > storing all 10 Ubuntu Jammy versions using PuzzleFS.
> > 
> > Here's a summary:
> > # Steps
> > 
> > * I’ve downloaded 10 versions of Jammy from hub.docker.com
> > * These images only have one layer which is in tar.gz format
> > * I’ve built 10 equivalent puzzlefs images
> > * Compute the tarball_total_size by summing the sizes of every Jammy
> >    tarball (uncompressed) => 766 MB (use this as baseline)
> > * Sum the sizes of every oci/puzzlefs image => total_size
> > * Compute the total size as if all the versions were stored in a single
> >    oci/puzzlefs repository => total_unified_size
> > * Saved space = tarball_total_size - total_unified_size
> > 
> > # Results
> > (See [5] if you prefer the video format)
> > 
> > | Type | Total size (MB) | Average layer size (MB) | Unified size (MB) | Saved (MB) / 766 MB |
> > | --- | --- | --- | --- | --- |
> > | Oci (uncompressed) | 766 | 77 | 766 | 0 (0%) |
> > | PuzzleFS uncompressed | 748 | 74 | 130 | 635 (83%) |
> > | Oci (compressed) | 282 | 28 | 282 | 484 (63%) |
> > | PuzzleFS (compressed) | 298 | 30 | 53 | 713 (93%) |
> > 
> > Here's the script I used to download the Ubuntu Jammy versions and
> > generate the PuzzleFS images [6] to get an idea about how I got to these
> > results.
> > 
> > Can we achieve these results with the current erofs features?  I'm
> > referring specifically to this comment: "EROFS already supports
> > variable-sized chunks + CDC" [7].
> 
> Please see
> https://erofs.docs.kernel.org/en/latest/comparsion/dedupe.html

Great, I see you've used the same example as I did. Though I must admit
I'm a little surprised there's no mention of PuzzleFS in your document.

> 
> 	                Total Size (MiB)	Average layer size (MiB)	Saved / 766.1MiB
> Compressed OCI (tar.gz)	282.5	28.3	63%
> Uncompressed OCI (tar)	766.1	76.6	0%
> Uncomprssed EROFS	109.5	11.0	86%
> EROFS (DEFLATE,9,32k)	46.4	4.6	94%
> EROFS (LZ4HC,12,64k)	54.2	5.4	93%
> 
> I don't know which compression algorithm are you using (maybe Zstd?),
> but from the result is
>   EROFS (LZ4HC,12,64k)  54.2
>   PuzzleFS compressed   53?
>   EROFS (DEFLATE,9,32k) 46.4
> 
> I could reran with EROFS + Zstd, but it should be smaller. This feature
> has been supported since Linux 6.1, thanks.

The average layer size is very impressive for EROFS, great work.
However, if we multiply the average layer size by 10, we get the total
size (5.4 MiB * 10 ~ 54.2 MiB), whereas for PuzzleFS, we see that while
the average layer size is 30 MIB (for the compressed case), the unified
size is only 53 MiB. So this tells me there's blob sharing between the
different versions of Ubuntu Jammy with PuzzleFS, but there's no sharing
with EROFS (what I'm talking about is deduplication across the multiple
versions of Ubuntu Jammy and not within one single version).

Of course, with only 10 images, the space savings don't seem that
impressive for PuzzleFS compared to EROFS, but imagine we are storing
hundreds/thousands of Ubuntu versions. Then we're also building OCI
images on top of these versions. So if the user already has all the
blobs for an Ubuntu version, then we only need to ship the chunks that
have changed / have been added as a result of the specific application
that we've built on top of an existing Ubuntu version.

One more thing: the "Unified size" column is the key for understanding
the space savings offered by PuzzleFS and I see that you've left this
column out of your table.

Regards,
Ariel

> 
> Thanks,
> Gao Xiang