Re: Including object type and size in object id (Re: Git Merge contributor summit notes)

From: Jeff Hostetler <git@jeffhostetler.com>
To: Jonathan Nieder <jrnieder@gmail.com>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Alex Vandiver" <alexmv@dropbox.com>,
	git@vger.kernel.org, jonathantanmy@google.com, bmwill@google.com,
	stolee@gmail.com, sbeller@google.com, peff@peff.net,
	johannes.schindelin@gmx.de,
	"Michael Haggerty" <mhagger@alum.mit.edu>
Subject: Re: Including object type and size in object id (Re: Git Merge contributor summit notes)
Date: Mon, 26 Mar 2018 17:42:15 -0400	[thread overview]
Message-ID: <b7b6d617-1951-5934-5b1d-bb1a300006ef@jeffhostetler.com> (raw)
In-Reply-To: <20180326210039.GB21735@aiede.svl.corp.google.com>

On 3/26/2018 5:00 PM, Jonathan Nieder wrote:
> Jeff Hostetler wrote:
> [long quote snipped]
> 
>> While we are converting to a new hash function, it would be nice
>> if we could add a couple of fields to the end of the OID:  the object
>> type and the raw uncompressed object size.
>>
>> If would be nice if we could extend the OID to include 6 bytes of data
>> (4 or 8 bits for the type and the rest for the raw object size), and
>> just say that an OID is a {hash,type,size} tuple.
>>
>> There are lots of places where we open an object to see what type it is
>> or how big it is.  This requires uncompressing/undeltafying the object
>> (or at least decoding enough to get the header).  In the case of missing
>> objects (partial clone or a gvfs-like projection) it requires either
>> dynamically fetching the object or asking an object-size-server for the
>> data.
>>
>> All of these cases could be eliminated if the type/size were available
>> in the OID.
> 
> This implies a limit on the object size (e.g. 5 bytes in your
> example).  What happens when someone wants to encode an object larger
> than that limit?

I could say add a full uint64 to the tail end of the hash, but
we currently don't handle blobs/objects larger then 4GB right now
anyway, right?

5 bytes for the size is just a compromise -- 1TB blobs would be
terrible to think about...

> 
> This also decreases the number of bits available for the hash, but
> that shouldn't be a big issue.

I was suggesting extending the OIDs by 6 bytes while we are changing
the hash function.

> Aside from those two, I don't see any downsides.  It would mean that
> tree objects contain information about the sizes of blobs contained
> there, which helps with virtual file systems.  It's also possible to
> do that without putting the size in the object id, but maybe having it
> in the object id is simpler.
> 
> Will think more about this.
> 
> Thanks for the idea,
> Jonathan
> 

Thanks
Jeff