What are .commitmeta files and why is delta update so huge?

[Leandro Santiago <leandrosansilva gmail com>]

Hi, when running upgrade in the client using delta files, I noticed it
tries to request a .commitmeta file, which does not exist and I don't
know why and if it has any influence in the process.

To summarize, I have generated delta files from commit
2435da4b6874efcfeaae9e342842676fe8a590f89dc80b3b2c991e4f6c8af506 to
0c5135c92c25fd4c90c1d55b710480f5f87de79e141cdfe3b9370ce9aeb57a2e

$ ostree static-delta list
2435da4b6874efcfeaae9e342842676fe8a590f89dc80b3b2c991e4f6c8af506-0c5135c92c25fd4c90c1d55b710480f5f87de79e141cdfe3b9370ce9aeb57a2e

The client requests:
config -> OK
refs/heads/centos-atomic-egym/7/x86_64/dev -> OK
deltas/JD/XaS2h078_qrp40KEJnb+ilkPidyAs7LJkeT2yK9QY-DFE1ySwl_UyQwdVbcQSA9fh9554UHN_juTcM6a61ei4/superblock
-> OK (980KB)
deltas/JD/XaS2h078_qrp40KEJnb+ilkPidyAs7LJkeT2yK9QY-DFE1ySwl_UyQwdVbcQSA9fh9554UHN_juTcM6a61ei4/0
-> OK (200KB)
objects/0c/5135c92c25fd4c90c1d55b710480f5f87de79e141cdfe3b9370ce9aeb57a2e.commitmeta
-> NOT FOUND
objects/aa/70d6d08d6dd0e8e186131896a7ab150a102b938538cb7ef9ac83c21953c48f.filez
-> OK (17MB)
objects/a4/92a68fdba07c976ba35c8977e8b4bca243ed7b3b15c1506213c82ee75ba518.filez
-> OK (17MB)

The difference between the two versions is very small, just some
modifications on some txt files (this is intentional). Before using
deltas, those updates were 40MB in size, so I was expecting a drastic
reduce in size when using delta, but it reduced to 34MB, which I think
is still not compatible the delta update idea.

So I wonder why the .filez (are they part of delta stuff, each one is
17MB in size?) were downloaded, if the superblock and 0 file were
found. I suspect it's related to the lack of the .commitmeta file, but
I am not sure.

Yes, I know the delta system is not finished yet and you are working
on it (I've been checking the source code and I might be able to
contribute with something...), but I for now I'd like to know what is
the behavior I should expect so far.

Oh, when I commit a new tree, I use the parameter --generate-sizes,
and I do now know if it's related to the problem too.

Thanks in advance.

-- 
Sent from my mind