Re: Some problems for git beginners

[Philip Oakley <philipoakley@iee.email>]

Please don't drop the list...

On 11/10/2021 04:00, ZhangJinbao wrote:
>
>
>> 2021年10月9日 下午6:32，Philip Oakley <philipoakley@iee.email
>> <mailto:philipoakley@iee.email>> 写道：
>>
>> On 09/10/2021 10:12, Bagas Sanjaya wrote:
>>> On 09/10/21 14.43, ZhangJinbao wrote:
>>>
>>>> Third, when a commit event occurs, the current blob file stores the
>>>> full amount of data, while the previous version stores the
>>>> incremental data
>>>>
>>>
>>> When you `git commit`, Git will initially write objects related to the
>>> commit (blob, tree, and commit) as loose objects. Some point on the
>>> time, Git will repack many loose objects into one big packfile and
>>> write the corresponding pack index. Delta compression is applied when
>>> writing the packfile.
>>>
>> Git is multi-layered, and multi-faceted. So often the impression given
>> isn't the same as what happens internally.
>>
>> All commits are full snapshots, even if git (normally) shows only the
>> changes for previous diffs.
>>
>> But then git will, at a convenient point 'pack' all those snapshots
>> (fairly efficiently) so that the local git storage of the whole
>> repository, with full history, (typically) is no bigger than the checked
>> out files. The packing is part of that layering.
>>
>> There are similar facet/layer effects from the Staging area ("index") /
>> Object store viewpoints.
>>
>> The
>> https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Porcelain
>> <https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Porcelain>
>> is worth a read, along with other descriptions.
>>
>> Philip
>>
>
> I'm very glad to hear from you
>
> After further study and practice in recent days, there are still
> several questions to confirm
>
> As the article says: 
> What is also interesting is that the second version of the file is the
> one that is stored intact, whereas the original version is stored as a
> delta — this is because you’re most likely to need faster access to
> the most recent version of the file.
>
> Practice has proved that:
> Using the repo.rb(22KB) file, you can store the original version as a
> delta. But using the test.txt(22B) file, you cannot store the original
> version as a delta. 
> Does this algorithm depend on the size of the file? If so, what is the
> file size boundary?
>
> ——————————————————————————————————————————————————————————
> zhangjinbao@Jinbao test % ls   
> repo.rb
> zhangjinbao@Jinbao test % echo "version 1" > test.txt
> zhangjinbao@Jinbao test % git add .
> zhangjinbao@Jinbao test % git commit -m "Create test.txt"
> [master b4a4c9d] Create test.txt
>  1 file changed, 1 insertion(+)
>  create mode 100644 test.txt
> zhangjinbao@Jinbao test % echo "# version 2" >> test.txt 
> zhangjinbao@Jinbao test % git commit -am "modify test.txt"
> [master c454823] modify test.txt
>  1 file changed, 1 insertion(+)
> zhangjinbao@Jinbao test % git gc
> ...
> zhangjinbao@Jinbao test % git verify-pack -v
> .git/objects/pack/pack-17ceeb7d325a4eb7dab3f6f04dda67daa7515feb.idx 
> c45482367fb06750398339597be1c0679af8ee9a commit 226 157 12
> b4a4c9de03421e9e49eaae3c67cf43c699eb5b98 commit 226 157 169
> bcb782fa38f8b016b263a129d3960f409f285215 commit 231 161 326
> 228b55de49116a358ec7c15ccb291678a014fcc8 commit 175 124 487
> b042a60ef7dff760008df33cee372b945b6e884e blob   22054 5799 611
> d25a248172e09b2ef61bcf45b5ec29435da23241 blob   22 26 6410
> aef683d9d2619240c4fa1dcb5a04ea8cbe1de89a tree   71 79 6436
> fdd7ee840023b632768fdcdc08a3465a0c4d46ed tree   71 78 6515
> 83baae61804e65cc73a7201a7252750c76066a30 blob   10 19 6593
> a1ca41f02e3519c32aafb8f4d4d9f465c8ce587a tree   35 46 6612
> 38feecbdf638935287fd920e8f2d694aa8c28d9f tree   35 46 6658
> 033b4468fa6b2a9547a70d88d1bbe8bf3f9ed0d5 blob   9 20 6704 1
> b042a60ef7dff760008df33cee372b945b6e884e
> 非 delta：11 个对象
> 链长 = 1: 1 对象
> .git/objects/pack/pack-17ceeb7d325a4eb7dab3f6f04dda67daa7515feb.pack: ok
> zhangjinbao@Jinbao test % 
>
> --zhangjinbao

The packing is a heuristic, which is explained in narrative (story) form
here: https://git-scm.com/docs/pack-heuristics
--
Philip