we alrady have the concept of objects that contain objects and therefor don'e need to be re-checked (directories), the chunks inside a file could be the same type of thing.

currently we say that if the hash on the directory is the same we don't need to re-check each of the files in that directory, this would be that if the hash on the file hasn't changed we don't need to re-check the chunks inside that file.

David Lang

On Fri, 15 Apr 2005, Ray Heasman wrote:

Date: Fri, 15 Apr 2005 12:33:03 -0700
From: Ray Heasman <[EMAIL PROTECTED]>
To: git@vger.kernel.org
Subject: Re: space compression (again)

For for this email not threading properly, I have been lurking on the
mail list archives and just had to reply to this message.

I was planning to ask exactly this question, and Scott beat me to to. I
even wanted to call them "chunks" too. :-)

It's probably worthwhile for anyone discussing this subject to read this
link: http://www.cs.bell-labs.com/sys/doc/venti/venti.pdf . I know it's
been posted before, but it really is worth reading. :-)

On Fri, 15 Apr 2005, Linus Torvalds wrote:
On Fri, 15 Apr 2005, C. Scott Ananian wrote:

Why are blobs per-file? [After all, Linus insists that files are an illusion.] Why not just have 'chunks', and assemble *these* into blobs (read, 'files')? A good chunk size would fit evenly into some number of disk blocks (no wasted space!).

I actually considered that. I ended up not doing it, because it's not obvious how to "block" things up (and even more so because while I like the notion, it flies in the face of the other issues I had: performance and simplicity).

I don't think it's as bad as you think.

Let's conceptually have two types of files - Pobs (Proxy Objects, or
Pointer Objects), and chunks. Both are stored and referenced by their
content hash, as usual. Pobs just contain a list of hashes referencing
the chunks in a file. When a file is initially stored, we chunk it so
each chunk fits comfortably in a block, but otherwise we aren't too
critical about sizes. When a file is changed (say, a single line edit),
we update the chunk that contains that line, hash it and store it with
its new name, and update the Pob, which we rehash and restore. If a
chunk grows to be very large (say > 2 disk blocks), we can rechunk it
and update the Pob to include the new chunks.

The problem with chunking is:
 - it complicates a lot of the routines. Things like "is this file
   unchanged" suddenly become "is this file still the same set of chunks",
   which is just a _lot_ more code and a lot more likely to have bugs.

You're half right; it will be more complex, but I don't think it's as bad as you think. Pobs are stored by hash just like anything else. If some chunks are different, the pob is different, which means it has a different hash. It's exactly the same as dealing with changed file now. Sure, when you have to fetch the data, you have to read the pob and get a list of chunks to concatenate and return, but your example given doesn't change.

 - you have to find a blocking factor. I thought of just going it fixed
   chunks, and that just doesn't help at all.

Just use the block size of the filesystem. Some filesystems do tail packing, so space isn't an issue, though speed can be. We don't actually care how big a chunk is, except to make it easy on the filesystem. Individual chunks can be any size.

 - we already have wasted space due to the low-level filesystem (as
   opposed to "git") usually being block-based, which means that space
   utilization for small objects tends to suck. So you really want to
   prefer objects that are several kB (compressed), and a small block just
   wastes tons of space.

If a chunk is smaller than a disk block, this is true. However, if we size it right this is no worse than any other file. Small files (less than a block) can't be made any larger, so they waste space anyway. Large files end up wasting space in one block unless they are a perfect multiple of the block size.

When we increase the size of a chunk, it will waste space, but we would
have created an entire new file, so we win there too.

Admittedly, Pobs will be wasting space too.

On the other hand, I use ReiserFS, so I don't care. ;-)

 - there _is_ a natural blocking factor already. That's what a file
   boundary really is within the project, and finding any other is really
   quite hard.

Nah. I think I've made a good case it isn't.

So I'm personally 100% sure that it's not worth it. But I'm not opposed to
the _concept_: it makes total sense in the "filesystem" view, and is 100%
equivalent to having an inode with pointers to blocks. I just don't think
the concept plays out well in reality.

Well, the reason I think this would be worth it is that you really win when you have multiple parallel copies of a source tree, and changes are cheaper too. If you store all the chunks for all your git repositories in one place, and otherwise treat your trees of Pobs as the real repository, your copied trees only cost you space for the Pobs. Obviously this also applies for file updates within past revisions of a tree, but I don't know how much it would save. It fits beautifully into the current abstraction, and saves space without having to resort to rolling hashes or xdeltas.

The _real_ reason why I am excited about git is that I have a vision of
using this as the filesystem (in a FUSE wrapper or something) for my
home directory. MP3s and AVIs aside, it will make actual work much
easier for me. I have a dream; a dream where I save files using the same
name, safe in the knowledge that I can get to any version I want. I will
live in a world of autosaves, deletes without confirmation, and /etcs
immune from the vagaries of my package management systems, not to
mention users not asking me leading questions about backups. *sigh*
*sniff* Excuse me, I think I have to go now.


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