Re: Why is the name of a blob SHA1("$type $size\0$data") and not SHA1("$data")?

From: Björn Steinbrink <b.steinbrink@gmx.de>
Date: Apr 30, 2009, 22:57 UTC
Message-ID: <20090430225733.GC18940@atjola.homenet>
In-Reply-To: <49FA0214.70009@gmail.com>

On 2009.04.30 20:55:00 +0100, David Srbecky wrote:

Show 19 quoted lines

> Hi,
>
>
> First of all, congratulations on makeing such a great version control  
> system.  I love the storage model - in comparison with other systems, it  
> is just birantly simple and ingenious.
>
>
> I started digging into the details and there is one thing that is really  
> bugging me - why is the name of a blob SHA1("$type $size\0$data") and  
> not SHA1("$data")?  I mean, wouldn't it be beautiful if the name of the  
> blob would really just be the SHA1 of the uncompressed file content? :-)
>
>
> Furthermore, is the header really necessary?  Wouldn't it be  
> eqvivalently effective to put the blobs into own subdirectory? For  
> example:  .git\objects\blob\22\22a3d28c5b2fca0eae83be1a2ed619e357f6a1e6
> So the blob would contatin just be the compressed content and nothing  
> else - beautiful :-)

Yes, at least the type is pretty important. Consider just "git show $some_object_name". If the object name was just the hash of the contents, you could have a blob and a commit with the same name. Which is which? And which do you mean in that command? The command line interface would need to accept a type in addition to the object name in a lot of place.

And in packs, you want the objects ordered so that you get could access patterns, and don't read from all over the pack file. That means that you would need the type header there, regardless of whether it is in the loose object file.

Björn