CODEWITHSUNDEEP
githasharchitectureblob
Ark-kun
Ark-kun

Reputation: 6787

Why does Git store (and hash) blob size in the blob file?

Git's blob object file format is blob <size string>\0<data>. The blob-identifying SHA-1 hash is calculated not from the blob contents alone, but from the header-augmented blob data (as described above).

As a purist I do not like that architecture. It mixes the universal property of the data (its SHA1 hash) with some git-specific header.

Another advantage of pure-data blob storage is that the files can be added to the index using "copy-on-write" instead of copying the whole file. The required space could be halved and some operations could become faster.

So, why did Git developers choose to use the header-based format instead of the pure data format?

P.S. AFAIK in the early days of Git the SHA-1 hash was based on the compressed data.

Upvotes: 6

Views: 768

Answers (1)

VonC
VonC

Reputation: 1324576

AFAIK in the early days of Git the SHA-1 hash was based on the compressed data.

Yes, and that lead to all kind of "optimizations" like commit 65c2e0c, git 0.99, June 2015:

Find size of SHA1 object without inflating everything.

But that new format, illustrated in "How does git compute file hashes?", can be traced back to:

Each time, the length of the data is needed to do anything with the data itself.

Upvotes: 5

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